Sampling

ABSTRACT

There is disclosed current-mode time-interleaved sampling circuitry configured to be driven by substantially sinusoidal clock signals. Such circuitry may be incorporated in ADC circuitry, for example as integrated circuitry on an IC chip. The disclosed circuitry is capable of calibrating itself without being taken off-line.

The present invention relates to circuitry and methods for sampling aninput signal, for example for sampling an input analogue signal. Inparticular, but not exclusively, the present invention relates toanalogue-to-digital circuitry and methods.

Architectures for realising analogue-to-digital converters (ADCs)generally fall into one of three categories, namely low-to-medium speed(e.g. integrating and oversampling ADCs), medium speed (e.g. algorithmicADCs) and high speed (e.g. time-interleaved ADCs). Typically, as thespeed of ADCs increase, their accuracy is found to fall.

The main idea behind time-interleaved ADCs is to obtain very-high-speedanalogue-to-digital conversion by operating many sub-ADCs in parallel.Examples of known time-interleaved ADCs are disclosed in“Time-Interleaved Converter Arrays” by W. C. Black, Jr. and D. A.Hodges, IEEE Journal of Solid-State Circuits, Volume 15, pages 1022 to1029, December 1980, and in “A 1-GHz 6-bit ADC System”, by K. Poulton,J. J. Corcoran, and T. Hornak, IEEE Journal of Solid-State Circuits,Volume 22, pages 962 to 970, December 1987.

FIG. 1 is a schematic diagram of a time-interleaved ADC 1 which may beconsidered generally equivalent to the known examples cited above. ADC 1comprises a first sample-and-hold (S/H) circuit 2, four second S/Hcircuits 4, four N-bit ADCs 6 and a digital multiplexer 8.

The first S/H circuit 2 is controlled by clock θ₀ and the second S/Hcircuits 4 are controlled by clocks θ₁ to θ₄. Clock θ₀ is four times asfast as clocks θ₁ to θ₄ and clocks θ₁ to θ₄ are delayed relative to oneanother by the period of clock θ₀. Thus, the ADCs 6 receive samples ofthe input signal V_(IN) sampled at the rate of clock θ₀ in turn, so thatthe samples received by one ADC 6 are time-interleaved with samplesreceived by the other ADCs 6. The benefit of this approach is that thefour ADCs 6 can operate at a quarter of the rate of the input samplingfrequency.

The digital outputs of the ADCs 6 are multiplexed together in thedigital multiplexer 8 and output as a digital signal representative ofthe original analogue input signal V_(IN) input to the first S/H circuit2.

Clearly, the first S/H circuit 2 is critical (because of the frequencyat which it must operate), whereas it is possible to tolerateconsiderable jitter (phase noise) in the four second S/H circuits 4because the samples have already been taken at that point. As a result,the first S/H circuit 2 is sometimes realised off-silicon for improvedperformance, for example using GaAs technology.

An improved time-interleaved ADC is disclosed in “A 24 GS/s 6b ADC in 90nm CMOS”, by Peter Schvan et al, ISSCC 2008, Session 30, Data-ConverterTechniques, 30.3, 2008 IEEE International Solid-State CircuitsConference, pages 544, 545, and 634. FIG. 2 is a schematic diagram of atime-interleaved ADC 10, which may be considered generally equivalent tothe ADC disclosed in this ISSCC paper.

The ADC 10 comprises four track-and-hold (T/H) circuits, four ADC banks14, four demultiplexers 16, and a digital multiplexer 18. The overallarchitecture of the ADC 10 is comparable to that of ADC 1. In this way,the T/H circuits 12 operate in a time-interleaved fashion and at aquarter of the overall sampling frequency.

ADC banks 14 are employed to convert the received samples into digitalsignals. ADC banks 14 may each comprise a plurality of time-interleavedsub-ADC circuits so as to enable those ADC circuits to operate at a yetslower rate than the T/H circuits 12, thereby demultiplexing each of theincoming signals into a plurality of output signals. Furtherdemultiplexing may be carried out by demultiplexers 16. The digitaloutputs of the demultiplexers 16 are finally multiplexed together in thedigital multiplexer 18 and output as a digital signal representative ofthe original analogue input signal V_(IN) input to the T/H circuits 12.

It will therefore be appreciated that the existing architecture for anultra-high-speed ADC such as ADC 10 of FIG. 2 is an array of interleavedT/H circuits to take initial samples, followed bydemultiplexing/sample-and-hold circuits which drive an array oflow-speed sub-ADCs. This existing ADC circuitry is voltage-modecircuitry.

One problem with this architecture is that it is particularly difficult(if not impossible) to obtain gain and skew (sample time) matchingbetween the different front-end T/H channels, gain matching between thedifferent demultiplexer/sample-and-hold channels andgain/linearity/offset matching between the sub-ADCs. Any mismatchesappear as spurs and images in the frequency domain or as fixed-patternednoise in the time domain. Even if some means of trimming the samplingdelays can be provided, the difficulty of measuring these delay errorsremains, because those errors are far too small to be measured directly.

A further problem is in achieving enough speed and linearity out of thefront end T/H circuits, because these need to have very high bandwidthand low distortion. The issue is not just static linearity but alsodynamic linearity in the sampling switches, because any signal-dependentswitching delay introduces distortion on high-frequency input signals.

Another major problem is that to get high speed, low capacitance andlower power in the sampling gates, very small devices must be used,which means that random mismatches are very large and difficult tocompensate for. It is also difficult to get good input matching at highfrequencies, and there is significant signal-dependent switchingkickback which causes distortion.

To provide an idea of the design difficulty involved, to maintain anENOB (Effective Number of Bits) of 6 bits for a 20 GHz input signal, theT/H sampling times need to be interleaved to within 0.1 ps accuracy ofthe ideal sample instance. This is impossible to do by design, socalibration is needed. However, the times cannot be measured directlywith enough accuracy, e.g. by measuring clock-delay differences, so acalibration signal generally needs to be applied to enable the timing tobe tuned (in very small steps) by minimizing FFT spurs. Even if this canbe done, generating and distributing the multiple high-speed samplingclocks and providing fine-delay time adjustments is also difficult andconsumes a lot of power.

For a better understanding, the above problems in existingtime-interleaved ADCs such as ADC 10 of FIG. 2 will be considered inmore detail.

FIG. 3 is a schematic diagram of circuitry 20 which may be employed inT/H circuits 12 of ADC 10. Circuitry 20 comprises a resistance 21, atransistor 22, a clock buffer 24, a signal buffer 26, and a capacitor28. Typically, an analogue input voltage V_(IN) is sampled by beinggated through the transistor 22 under control of a clock signal CLK, andthis sampled voltage is temporarily stored on capacitor 28 before beingemployed by the rest of the circuitry via buffer 26.

A desire to operate such circuitry at a high sample rate placesconsiderable pressure on its operation. For example, in order to operateat a desired sample rate of 56 GS/s the width of the clock pulses mayneed to be in the order of 17 ps (as indicated in FIG. 3).

Such fast operation also necessitates a small transistor 22 (e.g. havinga gate area of less than 1 square micron), and a small capacitor 28(e.g. having a capacitance C in the order of a few fF). At these smallsizes, mismatch in the transistors 22 (for example, in the parasiticcapacitances and on-resistances R_(ON)), and in the capacitors 28between the different sampling circuits 20 of the ADC, becomessignificant.

Moreover, resistance 21 is generally provided to give a desired inputimpedance (e.g. 50Ω), and to maintain good input matching the impedanceof the capacitor 28 must be much higher (e.g. 10 times) than that of theresistance 21 at the frequency of interest. For example, at 20 GHz foran impedance of 500Ω, the capacitor 28 may need to have a capacitance inthe region of 16 fF. At this tiny size, mismatches between thecapacitors 28 of the T/H circuits can become very significant.

Furthermore, with such a high clock rate, the slew rate becomessignificant. FIG. 4 is an enlarged diagram of a clock signal as it fallsfrom its high level V_(DD) (digital “1”) to its low level V_(SS)(digital “0”). Also shown in FIG. 4 is an example input signal V_(IN)fluctuating between maximum and minimum signal values. Although thesignals in FIG. 4 are not shown to scale, it will be appreciated thatover the range of possible input values of the signal V_(IN) (themaximum, middle-point and minimum values are indicated by black dots),there will be a corresponding range in values of the clock signal CLK atwhich the transistor 22 will switch (again, the corresponding maximum,middle and minimum such values are indicated by black dots). This isbecause the switching point of a transistor depends on the gate-sourcevoltage V_(GS) which is dependent upon V_(CLK)−V_(IN). This range inclock signal CLK values leads to an input-signal (V_(IN)) dependentrange of switching timing errors ΔT as shown in FIG. 4.

FIG. 5 is a diagram useful for further understanding the effect of suchslew-rate-related and input-dependent timing errors. The clock signalCLK is shown as a perfect sinewave, together with a schematic indicationof the clock skew on the falling edges thereof (the falling edge isassumed to be critical and, for simplicity, slew on the rising edge isnot indicated).

Along the input signal (V_(IN)) waveform shown in the lower half of FIG.5, black dots represent the intended sample points corresponding to thefalling clock edges in the clock waveform above. However, it will beappreciated by comparison with FIG. 4, and assuming that the circuitryis designed to take samples correctly when the input signal V_(IN) is atthe mid-point of its range, that the samples taken when V_(IN) is in thepositive half of its range will be taken early by an input-signaldependent amount and, conversely, that the samples taken when V_(IN) isin the negative half of its range will be taken late, again by aninput-signal dependent amount. Clock skew, phase noise (jitter),amplitude noise, and mismatching of transistors and capacitors compoundthese problems, leading to high distortion, i.e. a low ENOB, in theeventual ADC output.

The effects of such mismatches, and such clock slew, skew and jitter,can be summarized as causing delay mismatch (mainly caused by the clockgeneration and sampling circuitry), gain mismatch (mainly caused by thesampling and demultiplexing circuitry) and roll-off or bandwidthmismatch (again, mainly caused by the sampling and demultiplexingcircuitry).

FIG. 6 is a frequency-response diagram indicating the effects of gainmismatch G and roll-off mismatch R. Such mismatches can mean that theoverall frequency response of the ADC is unpredictable.

Another issue with the FIG. 2 ADC architecture relates to the issue ofcalibration. It is theoretically possible to apply a known test signalto the ADC and to examine the output of the ADC to aid calibration. Theoutput signal may for example be examined by performing an FFT and thenexamining the resulting frequency response. For example, with thedesired 56 GS/s ADC in mind, FIG. 7 shows, in its upper half, an exampleideal output FFT trace given an overall ADC sampling frequency of 56 GHzand therefore an overall ADC Nyquist frequency of 28 GHz. For thisexample, four-way sampler interleaving is assumed, with each sampleroperating at a sampling frequency of 14 GHz, i.e. one quarter of 56 GHz.

A test signal at 1 GHz is assumed to be applied to the input of the ADCin the absence of any other signal. Accordingly, a trace at 1 GHz isseen in the upper graph of FIG. 7, together with images thereof aboutthe 14, 28, 42 and 56 GHz frequencies, as would be expected.

Calibration can thus theoretically be performed by applying the testsignal and carrying out calibration to achieve the desired frequencyresponse. However, FFT processing is costly in terms of processing time,power consumption and complexity. Furthermore, during operation thesample circuits do not receive signals with a simple frequency responseas in the upper graph for FIG. 7. Instead, an input signal duringoperation will have a frequency spectrum as shown in the lower graph ofFIG. 7. Performing calibration during operation could thus beprohibitively complex, forcing calibration to be carried out whilst theADC is off-line. It is undesirable to need to take an ADC (for examplein use in an Ethernet connection) off-line to perform calibration.

A paper entitled “High-Speed Low-Power Integrating CMOS Sample-and-HoldAmplifier Architecture”, by L. Richard Carley and Tamal Muckherjee, IEEE1995 Custom Integrated Circuits Conference, pages 543 to 546, disclosesa “current-mode” sampling architecture for sample-and-hold (S&H)amplifiers which results in a reduction in error due to sampling clockjitter and aperture time. These reduced errors make possible a reductionin power over a conventional “voltage-mode” S&H with the same samplerate. This sampler uses switched logic-level clocks to drive thecircuitry in an attempt to get “square” ideal sampling pulses. It alsouses a transconductance stage to perform V-I conversion, which causesthe circuitry to suffer from noise and distortion. The input impedanceof the transconductor gate is capacitive, which means thatinput-matching performance of the circuitry is poor.

It is desirable to solve one or more of the above-mentioned problems. Itis desirable to provide sampling and analogue-to-digital conversioncircuits, methods and techniques which enable one or more of improveddistortion performance, reduced power consumption, reduced circuit size,and increased sampling rate.

According to an embodiment of a first aspect of the present invention,there is provided current-mode circuitry for sampling a current signal,the circuitry comprising: a first node configured to have the currentsignal applied thereto; X second nodes conductively connectable to saidfirst node along respective paths; and steering means for controllingsuch connections between the first node and the second nodes so thatdifferent packets of charge making up said current signal are steeredalong different said paths over time, wherein: X is an integer greaterthan or equal to 3; and the steering means comprises: control-signalgenerating means configured to generate X time-interleaved sinusoidalcontrol signals; and switching means distributed along said paths andconfigured to carry out such control in dependence upon the X sinusoidalcontrol signals.

Embodiments of the present invention carry out sampling based uponcurrents, i.e. in the current domain. That is, in embodiments of thepresent invention, any information content stored in the signals isrepresented by the current level of those signals. In the operation ofsuch circuitry, current is the independent variable and voltage isdependent on the current; current is the cause and voltage is theeffect. Such embodiments may be incorporated into, or employed inconjunction with, voltage-mode circuitry.

When X (where X>3) paths are controlled with a corresponding Xtime-interleaved control signals, it may (with previously-consideredcircuitry in mind) be considered appropriate to employ switchedlogic-level clock signals as those control signals. Such clock signalsmay for example be considered hard-switched clock signals orsquare-wave-type clock signals. In that case, it would be necessary toemploy clock signals each having a 100/X % duty cycle to achieve therequired 100/X % duty-cycle pulses to control the X paths. For example,when X=4, it would be necessary to employ clock signals each having a25% duty cycle to achieve the required 25% duty-cycle pulses to controlthe four paths. The use of such control signals may mean that thecircuitry is difficult if not impossible to use at high frequency (e.g.at 10 GHz+).

In contrast, in embodiments of the present aspect of the presentinvention, the X time-interleaved control signals are sinusoidal controlsignals. As a result, it becomes possible for 100/X % duty-cycle pulsesto be provided for controlling the X paths, but with each control signalitself having a shape typical of a 50% duty cycle (as is naturally trueof sinusoidal signals). For example, when X=4 it becomes possible toemploy 4 time-interleaved sinusoidal control signals, each having a 50%duty cycle, to form 25% duty-cycle pulses to control the four paths. Byusing the sinusoidal control signals, the pressures on control-signalgeneration are relaxed, which is particularly advantageous whenconsidering high-frequency operation. At a particular frequency ofoperation (e.g. sample rate, which may be the rate at which such packetsof charge are generated), a square-wave-type switched logic-level clocksignal requires much higher frequency components than the sinusoidalcontrol signals of the present aspect. Put another way, at a particularfundamental frequency of control signal, a sinusoidal signal (e.g. araised cosine signal) has a low slew rate and is the only signal thatonly has content at the fundamental frequency.

The following are optional features relating to the present aspect,however those features may be provided in combination with features ofthe other circuitry aspects disclosed herein.

The control signals may be raised generally cosine signals. For example,the shape of the signals may only need to be close to that of a perfectraised cosine signal in the upper part of the signals. The controlsignals may be substantially evenly time-interleaved relative to oneanother.

In an advantageous embodiment, X may equal 4. This may, for example,enable ease of compatibility with a quadrature VCO (a convenientgenerator of the control signals).

The steering means may be configured such that each of the X paths iscontrolled by a different one of said X control signals. For example,the steering means may be configured such that each of the X paths issubstantially conductive when its control signal is at or around itsmaximum value, and is substantially non-conductive when its controlsignal is at or around its minimum value. The steering means may beconfigured such that when two of said control signals have the samevalue their corresponding paths have substantially the same level ofconductivity.

The switching means may comprise a transistor per path, and, for eachpath, the transistor concerned may be connected such that its channelforms part of that path and such that it is controlled by the controlsignal for that path. This may provide a convenient way to control theconductivity of the paths. The transistors may be MOSFETs having gateterminals, and, for each path, the gate terminal of the transistorconcerned may be connected to receive the control signal for that path.

The circuitry may further comprise an input node operable to receive aninput voltage signal, and conversion means connected between the inputnode and the first node for converting the input voltage signal into thecurrent signal, the conversion means being configured as passivecircuitry comprising a resistance for performing the conversion. Thismay advantageously enable the circuitry to be supplied a voltage signal(which is a more convenient input signal), despite it operating incurrent mode.

When performing V-I conversion in order to convert a supplied inputvoltage signal into a required current signal, it may be consideredappropriate (with previously-considered circuitry in mind) to employ atransconductance stage (i.e. an active V-I conversion circuit). Atransconductance stage can be useful for providing a true V-Iconversion, for example even if there is voltage ripple at the node towhich the current signal is to be applied. However, the input impedanceof such a transconductance stage tends to be capacitive (transconductorgate), and thus has a bandwidth limit in its operation. The use of sucha transconductance can thus be highly problematic at high frequency(e.g. at 10 GHz+). At high frequency, a transconductance stage canbecome a significant noise and distortion contributor, and it can becomevery difficult to achieve satisfactory input matching.

In embodiments of the present aspect of the present invention, aresistance is provided between the input node and the first node as partof passive conversion means, to convert the input voltage signal to thecurrent signal. This is in contrast to the above previously-consideredthinking that it is appropriate to employ an active transconductancestage. Such passive conversion means may be configured to have aresistive input impedance, and thus the passive V-I conversion stage ofthe present aspect may be employed at very high frequencies withoutcontributing significant noise or distortion, as constant inputimpedance can be achieved. In practical embodiments of the presentaspect, it has been found that voltage ripple at the first node is smallenough to still enable a sufficiently high degree of linearity to beachieved.

The conversion means may comprise a resistor network having theresistance and configured such that the input node has substantiallyconstant input impedance over frequency. The first node may have acapacitive input impedance, for example due to parasitic capacitance onthat node, and the network may comprise an inductance configured tocompensate for the capacitive input impedance at the first node.

The network may be configured such that an effect of a voltagefluctuation at said first node on conversion of the input voltage signalinto the current signal is scaled down. For example, the network mayhave a potential-divider configuration to perform such scaling down.

The circuitry may comprise means for generating sample values based upona characteristic of respective such charge packets passing through saidsecond nodes, the sample values being indicative of values of saidcurrent signal corresponding to the charge packets concerned. Such acharacteristic may, for example, be a peak value of the packets or anarea of (i.e. an amount of charge making up) the packets. Such samplevalues may, for example, be digital sample values, which may be outputto a digital signal processor (DSP), or other computing device.

The circuitry may comprise calibrating means operable to combine samplevalues from one or more of said second nodes to provide one or morecombined sample values, and to calibrate operation of the circuitry independence upon said combined sample value(s).

Over time, statistically speaking, it could be expected that acombination (e.g. an average) of the sample values passing through oneof the second nodes would be the same a such a combination of the samplevalues passing through another one of the second nodes. This assumesthat the current signal is carrying an information signal such as atypical data signal whose value is changing (pseudo-randomly) over time.This also assumes that the circuitry is operating correctly.

Thus, assuming that the current signal is indeed carrying such aninformation signal, then any differences between the combinations ofsample values as mentioned above, or between a combination and areference value, may be symptomatic of errors in the operation of thecircuitry, and such differences may advantageously be employed tocontrol operation of the circuitry to reduce or eliminate such errors(i.e. to calibrate operation of the circuitry).

Because such calibration makes use of “real” sample values, i.e. samplevalues generated from an applied information signal when the circuitryis in use, the calibration may be performed whilst the circuitry is inactive use, i.e. without taking it “off-stream” or “off-line” to performthe calibration.

The calibration means may be operable to combine sample values for asaid second node over time to create a combined sample value for thatnode, and to calibrate operation of the circuitry in dependence upon acomparison between that combined sample value and a reference value.Such combination may, for example, be a summation or an averagingcombination.

The calibration means may be operable to combine sample values for onesaid second node over time to create a combined sample value for thatnode, to combine sample values for another said second node over time tocreate a combined sample value for that other node, and to calibrateoperation of the circuitry in dependence upon a comparison between thosecombined sample values. The calibration means may be operable, for eachsaid second node, to combine sample values for that second node overtime to create a combined sample value, and to calibrate operation ofthe circuitry in dependence upon a comparison between those combinedsample values. Such comparisons may, for example, comprise looking fordifferences between said combined sample values, and looking at therelationships between those differences for different combinations ofsaid second nodes.

The calibration means may be operable to influence operation of thecontrol-signal generating means and/or the switching means so as toperform such calibration.

For example, the calibration means may be operable to control the phaseand/or magnitude of the control signals so as to perform saidcalibration. In one embodiment, the steering means is configured suchthat the conductive connection of each said second node to the firstnode is controlled by its said control signal; the calibration means isoperable to combine sample values for one candidate said second nodeover time to provide a combined sample value for that node, and tocombine sample values for another candidate said second node over timeto provide a combined sample value for that other node; and thecalibration means is operable to control the phase and/or magnitude ofthe control signal for one or both of those candidate nodes tocompensate for any difference between the combined sample values forthose candidate nodes.

As another example, the calibration means may be operable to control adependency of said switching means on said control signals so as toperform said calibration. In one embodiment, the switching meanscomprises a transistor per path; for each path, the transistor concernedis connected such that its channel forms part of that path and such thatit is controlled by the control signal for the second node of that path;the calibration means is operable to combine sample values for onecandidate said second node over time to provide a combined sample valuefor that node, and to combine sample values for another candidate saidsecond node over time to provide a combined sample value for that othernode; and the calibration means is operable to control a gate and/orbulk voltage for the transistor along the path for one or both of thosecandidate nodes to compensate for any difference between the combinedsample values for those candidate nodes.

The circuitry may comprise calibrating means operable analyse the orsome of the sample values and to calibrate operation of the circuitry independence upon such analysis.

In previously-considered circuitry, control signals for use by switchingmeans tend to be clock signals with fast clock edges, for exampleswitched logic-level clock signals such as square waves. In this way,the switching operations of the switching means are desensitised orshielded from the effects of mismatches between different switches ofthe switching means and to mismatches between different control signalsused to control the switching means.

In contrast, embodiments of the present aspect of the present inventionemploy sinusoidal control signals. The magnitude of a sinusoidal signalchanges very slowly over time from its maximum to its minimum. That is,for a control signal having a particular fundamental frequency, asinusoidal signal has a low slew rate and is the only signal that onlyhas content at the fundamental frequency. In this way, it is possible tomaximise the sensitivity of the switching operations of the switchingmeans to mismatches between different switches of the switching meansand to mismatches between different control signals used to control theswitching means. Put another way, by employing sinusoidal signals it maybe possible to maximise the gain from a change in mismatch to a changein the output sample value. This sensitivity may advantageously beemployed to calibrate operation of the circuitry to compensate for themismatches. That is, it may be possible to analyse the sample values andcalibrate operation based on the analysis.

The circuitry may be configured to have a tree structure, wherein: thefirst node is a root node of the tree structure; the second nodes arefirst-tier nodes of the tree structure each of which is conductivelyconnectable directly to the root node; the circuitry further comprises aplurality of subsequent-tier nodes of the tree structure per first-tiernode, each of which is conductively connectable indirectly to the rootnode along a respective path via its said first-tier node; and thesteering means is operable to control such connections between the rootnode and the subsequent-tier nodes so that different packets of chargemaking up said current signal are steered along different said pathsover time.

By arranging nodes in a tree structure (having a root node, first-tiernodes, and subsequent-tier-nodes per first-tier-node, with switchesmaking up the switching means distributed between the nodes along thepaths), it is possible to allow the specifications for the switchingperformed to become successively more relaxed from tier to subsequenttier. For example, assuming that the paths from the root node to thefirst-tier nodes are cycled through (for carrying packets) in an orderor sequence, and assuming that there are X first tier-nodes, then therate at which packets pass through a first-tier node is 1/X the rate atwhich packets pass through the root node. Further, extending thisexample through the next tier of nodes (part or all of the subsequenttier-nodes), then assuming that there are N second-tier nodes perfirst-tier node, the rate at which packets pass through a second-tiernode is 1/N the rate which packets pass through its first-tier node, and1/(X.N) the rate at which packets through the root node.

In one embodiment, the steering means is operable to employ saidsinusoidal control signals to control connections between the root nodeand the first-tier nodes; the control-signal generating means isoperable to generate a plurality of time-interleaved switched-logiccontrol signals; and the steering means is operable to employ theswitched-logic control signals to control connections between thefirst-tier nodes and the subsequent-tier nodes. The control signals usedto control connections between the first-tier of nodes and thesubsequent-tier of nodes may have larger peak-to-peak voltages and/orlonger on-times than that of the control signals used to controlconnections between the root node and the first-tier of nodes.

The generating means may be operable to generate sample values basedupon a characteristic of respective such charge packets passing throughsaid subsequent-tier nodes, the sample values being indicative of valuesof said current signal corresponding to the charge packets concerned.

The circuitry may comprise reset means connected to a said node otherthan the first node, and operable, during a period between packets forthat node, to tend to bring a voltage potential of that node to apredetermined value.

By bringing a voltage potential at a node to a predetermined value,during a period between packets for that node, it may be possible toreduce errors which may otherwise result from differing amounts ofresidual charge (for example, from previous charge packets) stored on aparasitic capacitance at that node. In one embodiment, the reset meansmay be operable to cause the voltage potential of that node to return tothe same value between successive packets for that node.

The circuitry may comprise a plurality of said reset means each for adifferent such node other than the first node. The may be a said resetmeans per each such node other than the first node. The value that thevoltage potential of each such node is caused to return to may bedifferent for each node, or substantially the same for each node.

The circuitry may be configured for sampling first and secondcomplementary said current signals. In one embodiment, the circuitrycomprises first and second complementary circuit portions, the firstportion for sampling the first current signal and the second portion forsampling the second current signal. Each such portion may comprise sucha first node, second nodes and steering means, and the circuitry mayfurther comprise reset means connected to at least one of a said secondnode of the first portion and the complementary node of the secondportion, and operable, during a period between packets for that pair ofcomplementary nodes, to tend to bring a difference in voltage potentialbetween those two nodes to a predetermined value.

By bringing a difference in the voltage potential between pairs ofcomplementary nodes to a predetermined value, during a period betweenpackets for those nodes, it may be possible to reduce errors which mayotherwise result from differing amounts of residual charge (for example,from previous charge packets) stored on parasitic capacitances of thosenodes.

The reset means may be connected to both nodes of the pair ofcomplementary nodes. The reset means may be operable to connect the orboth of the nodes of the pair of complementary nodes to (for example,via a capacitor) a reference potential, e.g. a ground supply, so as totend to bring the difference in potential to a predetermined value. Thereset means may be operable to bring the nodes of the pair ofcomplementary nodes to the same potential as one another. The resetmeans may be connected between the pair of complementary nodes, and maybe operable to connect those nodes together so as to bring those nodesto the same potential as one another. The reset means may be operable tocause a voltage potential of the or both (each) of the nodes of the pairof complementary nodes to return to the same value between successivepackets. There may be provided a plurality of such reset means each fora different such pair of complementary nodes.

Each portion may comprise generating means for generating sample valuesbased upon a characteristic of respective such charge packets passingthrough the second nodes of that portion, the sample values beingindicative of values of the current signal of that portion correspondingto the charge packets concerned.

The circuitry may be configured for sampling first and secondcomplementary said current signals. For example, the circuitry maycomprise first and second complementary circuit portions, the firstportion for sampling the first current signal and the second portion forsampling the second current signal. In one embodiment, each said portioncomprises such a first node, second nodes and steering means, each saidportion is configured in a tree structure, and, for each said portion,the first node is a root node of the tree structure, the second nodesare first-tier nodes of the tree structure each of which is conductivelyconnectable directly to the root node of that portion, and that portionfurther comprises a plurality of subsequent-tier nodes of its treestructure per first-tier node, each of which is conductively connectableindirectly to the root node of that portion along a respective path viaits said first-tier node. In such an embodiment, the steering means maybe operable to control such connections between the root node and thesubsequent-tier nodes so that different packets of charge making up saidcurrent signal are steered along different said paths over time. In suchan embodiment, the circuitry may further comprise reset means connectedto at least one of a said first or subsequent-tier node of the firstportion and the complementary node of the second portion, and operable,during a period between packets for that pair of complementary nodes, totend to bring a difference in potential between those two nodes to apredetermined value.

The reset means may be connected to both nodes of the pair ofcomplementary nodes. The reset means may be operable to connect the orboth of the nodes of the pair of complementary nodes to (for example,via a capacitor) a reference potential, e.g. a ground supply, so as totend to bring the difference in potential to a predetermined value. Thereset means may be operable to bring the nodes of the pair ofcomplementary nodes to the same potential as one another. The resetmeans may be connected between the pair of complementary nodes, and maybe operable to connect those nodes together so as to bring those nodesto the same potential as one another. The reset means may be operable tocause a voltage potential of the or both (each) of the nodes of the pairof complementary nodes to return to the same value between successivepackets. There may be provided a plurality of such reset means each fora different such pair of complementary nodes.

The circuitry may comprise generating means for generating sample valuesbased upon a characteristic of respective such charge packets passingthrough the subsequent-tier nodes of that portion, the sample valuesbeing indicative of values of the current signal of that portioncorresponding to the charge packets concerned.

According to an embodiment of a second aspect of the present invention,there is provided current-mode circuitry for sampling a current signal,the circuitry comprising: a first node configured to have the currentsignal applied thereto; a plurality of second nodes conductivelyconnectable to said first node along respective paths; steering meansfor controlling such connections between the first node and the secondnodes so that different packets of charge making up said current signalare steered along different said paths over time; generating means forgenerating sample values based upon a characteristic of respective suchcharge packets passing through said second nodes, the sample valuesbeing indicative of values of said current signal corresponding to thecharge packets concerned; and calibrating means operable to combinesample values from one or more of said second nodes to provide one ormore combined sample values, and to calibrate operation of the circuitryin dependence upon said combined sample value(s).

Such combination may be, for example, through summation or averaging.

Over time, statistically speaking, it may be expected that a combination(e.g. an average) of the sample values passing through one of the secondnodes would be the same a such a combination of the sample valuespassing through another one of the second nodes. This assumes that thecurrent signal is carrying an information signal such as a typical datasignal whose value is changing (pseudo-randomly) over time. This alsoassumes that the circuitry is operating correctly.

Thus, assuming that the current signal is indeed carrying such aninformation signal, then any differences between the combinations ofsample values as mentioned above, or between a combination and areference value, may be symptomatic of errors in the operation of thecircuitry, and such differences may advantageously be employed tocontrol operation of the circuitry to reduce or eliminate such errors(i.e. to calibrate operation of the circuitry).

Because such calibration makes use of “real” sample values, i.e. samplevalues generated from an applied information signal when the circuitryis in use, the calibration may be performed whilst the circuitry is inactive use, i.e. without taking it “off-stream” to perform thecalibration.

The following are optional features relating to the present aspect,however those features may be provided in combination with features ofthe other circuitry aspects disclosed herein.

The calibration means may be operable to combine sample values for asaid second node over time to create a combined sample value for thatnode, and to calibrate operation of the circuitry in dependence upon acomparison between that combined sample value and a reference value. Thecalibration means may be operable to combine sample values for one saidsecond node over time to create a combined sample value for that node,and to combine sample values for another said second node over time tocreate a combined sample value for that other node, and to calibrateoperation of the circuitry in dependence upon a comparison between thosecombined sample values. The calibration means may be operable, for eachsaid second node, to combine sample values for that second node overtime to create a combined sample value, and to calibrate operation ofthe circuitry in dependence upon a comparison between those combinedsample values. Such comparisons may, for example, comprise looking fordifferences between said combined sample values, and looking at therelationships between those differences for different combinations ofsaid second nodes.

The steering means may comprise: control-signal generating meansconfigured to generate a set of time-interleaved control signals, onefor each said second node; and switching means distributed along saidpaths and configured to carry out such control of the connections independence upon the control signals. In that case, the calibration meansmay be operable to influence operation of the control-signal generatingmeans and/or the switching means so as to perform such calibration.

The calibration means may be operable to control the phase and/ormagnitude of the control signals so as to perform said calibration. Inone embodiment, it may be that: the steering means is configured suchthat the conductive connection of each said second node to the firstnode is controlled by its said control signal; the calibration means isoperable to combine sample values for one candidate said second nodeover time to provide a combined sample value for that node, and tocombine sample values for another candidate said second node over timeto provide a combined sample value for that other node; and thecalibration means is operable to control the phase and/or magnitude ofthe control signal for one or both of those candidate nodes tocompensate for any difference between the combined sample values forthose candidate nodes.

The calibration means may be operable to control a dependency of saidswitching means on said control signals so as to perform saidcalibration. In one embodiment, it may be that: the switching meanscomprises a transistor per path; for each path, the transistor concernedis connected such that its channel forms part of that path and such thatit is controlled by the control signal for the second node of that path;the calibration means is operable to combine sample values for onecandidate said second node over time to provide a combined sample valuefor that node, and to combine sample values for another candidate saidsecond node over time to provide a combined sample value for that othernode; and the calibration means is operable to control a gate and/orbulk voltage for the transistor along the path for one or both of thosecandidate nodes to compensate for any difference between the combinedsample values for those candidate nodes.

According to an embodiment of a third aspect of the present invention,there is provided current-mode circuitry for sampling a current signal,the circuitry comprising: a first node configured to have the currentsignal applied thereto; a plurality of second nodes conductivelyconnectable to said first node along respective paths; steering meansfor controlling such connections between the first node and the secondnodes so that different packets of charge making up said current signalare steered along different said paths over time; and generating meansfor generating sample values based upon a characteristic of respectivesuch charge packets passing through said second nodes, the sample valuesbeing indicative of values of said current signal corresponding to thecharge packets concerned, wherein: the steering means comprisescontrol-signal generating means configured to generate substantiallysinusoidal control signals, and switching means distributed along saidpaths and configured to carry out such control in dependence upon thesinusoidal control signals; and the circuitry further comprisescalibrating means operable analyse the or some of the sample values andto calibrate operation of the circuitry in dependence upon suchanalysis.

In previously-considered circuitry, control signals for use by switchingmeans tend to be clock signals with fast clock edges, for exampleswitched logic-level clock signals such as square waves. In this way,the switching operations of the switching means are desensitised orshielded from the effects of mismatches between different switches ofthe switching means and to mismatches between different control signalsused to control the switching means.

In contrast, embodiments of the present aspect of the present inventionemploy sinusoidal control signals. The magnitude of a sinusoidal signalchanges very slowly over time from its maximum to its minimum. That is,for a control signal having a particular fundamental frequency, asinusoidal signal has a low slew rate and is the only signal that onlyhas content at the fundamental frequency. In this way, it is possible tomaximise the sensitivity of the switching operations of the switchingmeans to mismatches between different switches of the switching meansand to mismatches between different control signals used to control theswitching means. Put another way, by employing sinusoidal signals it ispossible to maximise the gain from a change in mismatch to a change inthe output sample value. This sensitivity may advantageously be employedto calibrate operation of the circuitry to compensate for themismatches. That is, it is possible to analyse the sample values andcalibrate operation based on the analysis.

The following are optional features relating to the present aspect,however those features may be provided in combination with features ofthe other circuitry aspects disclosed herein.

The calibration means may be operable to influence operation of thecontrol-signal generating means and/or the switching means so as toperform such calibration, as mentioned above.

According to an embodiment of a fourth aspect of the present invention,there is provided current-mode circuitry for sampling a current signal,the circuitry comprising: a root node configured to have the currentsignal applied thereto; a plurality of first-tier nodes each of which isconductively connectable directly to the root node; a plurality ofsubsequent-tier nodes per first-tier node, each of which is conductivelyconnectable indirectly to the root node along a respective path via itssaid first-tier node; and steering means for controlling suchconnections between the root node and the subsequent-tier nodes so thatdifferent packets of charge making up said current signal are steeredalong different said paths over time.

By arranging nodes in a tree structure (having a root node, first-tiernodes, and subsequent-tier-nodes per first-tier-node, with switchesmaking up the switching means distributed between the nodes along thepaths), it may be possible to allow the specifications for the switchingperformed to become successively more relaxed from tier to subsequenttier. For example, assuming that the paths from the root node to thefirst-tier nodes are cycled through (for carrying packets) in an orderor sequence, and assuming that there are X first tier-nodes, then therate at which packets pass through a first-tier node is 1/X the rate atwhich packets pass through the root node. Further, extending thisexample through the next tier of nodes (part or all of the subsequenttier-nodes), then assuming that there are N second-tier nodes perfirst-tier node, the rate at which packets pass through a second-tiernode is 1/N the rate which packets pass through its first-tier node, and1/(X.N) the rate at which packets through the root node.

The following are optional features relating to the present aspect,however those features may be provided in combination with features ofthe other circuitry aspects disclosed herein.

The steering means may be operable to employ sinusoidal control signalsto control connections between the root node and the first-tier nodes,and switched-logic control signals to control connections between thefirst-tier nodes and the subsequent-tier nodes. The control signals usedto control connections between the first-tier of nodes and thesubsequent-tier of nodes may have larger peak-to-peak voltages and/orlonger on-times than that of the control signals used to controlconnections between the root node and the first-tier of nodes.

Such circuitry may further comprise generating means operable togenerate sample values based upon a characteristic of respective suchcharge packets passing through said subsequent-tier nodes, the samplevalues being indicative of values of said current signal correspondingto the charge packets concerned.

According to an embodiment of a fifth aspect of the present invention,there is provided current-mode circuitry for sampling a current signal,the circuitry comprising: a first node configured to have the currentsignal applied thereto; a plurality of second nodes conductivelyconnectable to said first node along respective paths; steering meansfor controlling such connections between the first node and the secondnodes so that different packets of charge making up said current signalare steered along different said paths over time; and reset meansconnected to a said second node, and operable, during a period betweenpackets for that node, to tend to bring a voltage potential of that nodeto a predetermined value.

By bringing a voltage potential at a node to a predetermined value,during a period between packets for those nodes, it is possible toreduce errors which may otherwise result from differing amounts ofresidual charge (for example, from previous charge packets) stored on aparasitic capacitance at that node.

The following are optional features relating to the present aspect,however those features may be provided in combination with features ofthe other circuitry aspects disclosed herein.

The reset means may be operable to cause the voltage potential of thatsecond node to return to the same value between successive packets forthat node. The circuitry may comprise a plurality of such reset meanseach for a different such second node. There may be provided such areset means per second node.

According to an embodiment of a sixth aspect of the present invention,there is provided current-mode circuitry for sampling first and secondcomplementary current signals, the circuitry comprising: first andsecond complementary circuitry portions, the first portion for samplingthe first current signal and the second portion for sampling the secondcurrent signal, wherein each said portion comprises: a first nodeconfigured to have the current signal for that portion applied thereto;a plurality of second nodes conductively connectable to the first nodeof that portion along respective paths; and steering means forcontrolling such connections between the first node and the second nodesof that portion so that different packets of charge making up thecurrent signal of that portion are steered along different said paths ofthat portion over time; and reset means connected to at least one of asaid second node of the first portion and the complementary node of thesecond portion, and operable, during a period between packets for thatpair of complementary nodes, to tend to bring a difference in voltagepotential between those two nodes to a predetermined value.

By bringing a difference in voltage potential between pairs ofcomplementary nodes to a predetermined value, during a period betweenpackets for those nodes, it is possible to reduce errors which mayotherwise result from differing amounts of residual charge (for example,from previous charge packets) stored on parasitic capacitances of thosenodes.

The following are optional features relating to the present aspect,however those features may be provided in combination with features ofthe other circuitry aspects disclosed herein.

The reset means may be connected to both nodes of the pair ofcomplementary nodes. The reset means may be operable to connect the orboth of the nodes of the pair of complementary nodes to a referencepotential so as to tend to bring the difference in potential to apredetermined value. The reset means may be operable to bring the nodesof the pair of complementary nodes to the same potential as one another.The reset means may be connected between the pair of complementarynodes, and may be operable to connect those nodes together so as tobring those nodes to the same potential as one another. The reset meansmay be operable to cause a voltage potential of the or both of the nodesof the pair of complementary nodes to return to the same value betweensuccessive packets. There may be provided plurality of such reset meanseach for a different such pair of complementary nodes. There may beprovided such a reset means per such pair of complementary nodes.

In such circuitry, each said portion may comprises generating means forgenerating sample values based upon a characteristic of respective suchcharge packets passing through the second nodes of that portion, thesample values being indicative of values of the current signal of thatportion corresponding to the charge packets concerned.

According to an embodiment of a seventh aspect of the present invention,there is provided current-mode circuitry for sampling first and secondcomplementary current signals, the circuitry comprising: first andsecond complementary circuitry portions, the first portion for samplingthe first current signal and the second portion for sampling the secondcurrent signal, wherein each said portion comprises: a root nodeconfigured to have the current signal for that portion applied thereto;a plurality of first-tier nodes each of which is conductivelyconnectable directly to the root node of that portion; a plurality ofsubsequent-tier nodes per first-tier node of that portion, each of whichis conductively connectable indirectly to the root node of that portionalong a respective path via its said first-tier node; and steering meansfor controlling such connections between the root node and thesubsequent-tier nodes of that portion so that different packets ofcharge making up the current signal of that portion are steered alongdifferent said paths of that portion over time; and reset meansconnected to at least one of a said first or subsequent-tier node of thefirst portion and the complementary node of the second portion, andoperable, during a period between packets for that pair of complementarynodes, to tend to bring a difference in voltage potential between thosetwo nodes to a predetermined value.

By bringing a difference in voltage potential between pairs ofcomplementary nodes to a predetermined value, during a period betweenpackets for those nodes, it is possible to reduce errors which mayotherwise result from differing amounts of residual charge (for example,from previous charge packets) stored on parasitic capacitances of thosenodes.

The following are optional features relating to the present aspect,however those features may be provided in combination with features ofthe other circuitry aspects disclosed herein.

The reset means may be connected to both nodes of the pair ofcomplementary nodes. The reset means may be operable to connect the orboth of the nodes of the pair of complementary nodes to a referencepotential so as to tend to bring the difference in potential to apredetermined value. The reset means may be operable to bring the nodesof the pair of complementary nodes to the same potential as one another.The reset means may be connected between said pair of complementarynodes, and may be operable to connect those nodes together so as tobring those nodes to the same potential as one another. The reset meansmay be operable to cause a voltage potential of the or both of the nodesof the pair of complementary nodes to return to the same value betweensuccessive packets. There may be provided a plurality of such resetmeans each for a different such pair of complementary nodes. There maybe provided such a reset means per such pair of complementary nodes.

In such circuitry, each said portion may comprise: generating means forgenerating sample values based upon a characteristic of respective suchcharge packets passing through the subsequent-tier nodes of thatportion, the sample values being indicative of values of the currentsignal of that portion corresponding to the charge packets concerned.

According to an embodiment of an eighth aspect of the present invention,there is provided analogue-to-digital conversion circuitry, comprisingcircuitry according to any of the aforementioned aspects of the presentinvention. According to an embodiment of a ninth aspect of the presentinvention, there is provided integrated circuitry, comprising circuitryaccording to any of the aforementioned aspects of the present invention.According to an embodiment of a tenth aspect of the present invention,there is provided an IC chip, comprising circuitry according to any ofthe aforementioned aspects of the present invention.

According to an embodiment of an eleventh aspect of the presentinvention, there is provided a method of sampling a current signal incurrent-mode circuitry, the circuitry having a first node configured tohave the current signal applied thereto and X second nodes conductivelyconnectable to said first node along respective paths, the methodcomprising: controlling such connections between the first node and thesecond nodes in dependence upon X time-interleaved sinusoidal controlsignals so that different packets of charge making up said currentsignal are steered along different said paths over time, wherein: X isan integer greater than or equal to 3.

According to an embodiment of a twelfth aspect of the present invention,there is provided a method of sampling a current signal in current-modecircuitry, the circuitry having a first node configured to have thecurrent signal applied thereto and a plurality of second nodesconductively connectable to said first node along respective paths, themethod comprising: controlling such connections between the first nodeand the second nodes so that different packets of charge making up saidcurrent signal are steered along different said paths over time;generating sample values based upon a characteristic of respective suchcharge packets passing through said second nodes, the sample valuesbeing indicative of values of said current signal corresponding to thecharge packets concerned; combining sample values from one or more ofsaid second nodes to provide one or more combined sample values; andcalibrating operation of the circuitry in dependence upon said combinedsample value(s).

According to an embodiment of a thirteenth aspect of the presentinvention, there is provided a method of sampling a current signal incurrent-mode circuitry, the circuitry having a first node configured tohave the current signal applied thereto and a plurality of second nodesconductively connectable to said first node along respective paths, themethod comprising: generating substantially sinusoidal control signals;controlling such connections between the first node and the second nodesin dependence upon the sinusoidal control signals so that differentpackets of charge making up said current signal are steered alongdifferent said paths over time; generating sample values based upon acharacteristic of respective such charge packets passing through saidsecond nodes, the sample values being indicative of values of saidcurrent signal corresponding to the charge packets concerned; analysingthe or some of the sample values; and calibrating operation of thecircuitry in dependence upon such analysis.

According to an embodiment of a fourteenth aspect of the presentinvention, there is provided a method of sampling a current signal incurrent-mode circuitry, the circuitry having a root node configured tohave the current signal applied thereto, a plurality of first-tier nodeseach of which is conductively connectable directly to the root node, anda plurality of subsequent-tier nodes per first-tier node, each of whichis conductively connectable indirectly to the root node along arespective path via its said first-tier node, the method comprising:controlling such connections between the root node and thesubsequent-tier nodes so that different packets of charge making up saidcurrent signal are steered along different said paths over time.

According to an embodiment of a fifteenth aspect of the presentinvention, there is provided a method of sampling first and secondcomplementary current signals in current-mode circuitry, the circuitryhaving first and second complementary circuitry portions, the firstportion for sampling the first current signal and the second portion forsampling the second current signal, wherein each said portion comprisesa first node configured to have the current signal for that portionapplied thereto and a plurality of second nodes conductively connectableto the first node of that portion along respective paths, the methodcomprising: for each said portion, controlling such connections betweenthe first node and the second nodes of that portion so that differentpackets of charge making up the current signal of that portion aresteered along different said paths of that portion over time; and for atleast one of a said second node of the first portion and thecomplementary node of the second portion, and during a period betweenpackets for that pair of complementary nodes, tending to bring adifference in voltage potential between those two nodes to apredetermined value.

According to an embodiment of a sixteenth aspect of the presentinvention, there is provided a method of sampling first and secondcomplementary current signals in current-mode circuitry, the circuitryhaving first and second complementary circuitry portions, the firstportion for sampling the first current signal and the second portion forsampling the second current signal, wherein each said portion comprisesa root node configured to have the current signal for that portionapplied thereto, a plurality of first-tier nodes each of which isconductively connectable directly to the root node of that portion, anda plurality of subsequent-tier nodes per first-tier node of thatportion, each of which is conductively connectable indirectly to theroot node of that portion along a respective path via its saidfirst-tier node, the method comprising: for each said portion,controlling such connections between the root node and thesubsequent-tier nodes of that portion so that different packets ofcharge making up the current signal of that portion are steered alongdifferent said paths of that portion over time; and for at least one ofa said first or subsequent-tier node of the first portion and thecomplementary node of the second portion, and during a period betweenpackets for that pair of complementary nodes, tending to bring adifference in voltage potential between those two nodes to apredetermined value.

According to an embodiment of a seventeenth aspect of the presentinvention, there is provided a method of sampling a current signal incurrent-mode circuitry, the circuitry having a first node configured tohave the current signal applied thereto and a plurality of second nodesconductively connectable to said first node along respective paths, themethod comprising: controlling such connections between the first nodeand the second nodes so that different packets of charge making up saidcurrent signal are steered along different said paths over time; and fora said second node, and during a period between packets for that node,tending to bring a voltage potential of that node to a predeterminedvalue.

Features of apparatus (circuitry) aspects apply equally to methodaspects, and vice versa. Features of one circuitry aspect, such as thosefeatures specified as optional features, may be provided in combinationwith features of the other circuitry aspects.

The present invention extends to IC chips as mentioned above, circuitboards comprising such IC chips, and communication networks (forexample, internet fiber-optic networks and wireless networks) andnetwork equipment of such networks, comprising such circuit boards.

Reference will now be made, by way of example, to the accompanyingdrawings, of which:—FIG. 1, discussed above, is a schematic diagram of aknown time-interleaved ADC;

FIG. 2, discussed above, is a schematic diagram of another knowntime-interleaved ADC;

FIG. 3, discussed above, is a schematic diagram of T/H circuitry;

FIG. 4, discussed above, is an enlarged diagram of a clock signal, forappreciating the effect of slew rate;

FIG. 5, discussed above, is a diagram useful for understanding theeffect of slew-rate-related and input-dependent timing errors;

FIG. 6, discussed above, is a frequency-response diagram indicating theeffects of gain mismatch G and roll-off mismatch R;

FIG. 7, discussed above, presents an ideal FFT trace in its upper halfand an example frequency spectrum of an input signal in its lower half;

FIG. 8 is a schematic diagram of current-mode sampling circuitry;

FIG. 9 is a schematic diagram of analogue-to-digital circuitry embodyingthe present invention;

FIG. 10 is a schematic circuit diagram of a four-phase current-modesampler embodying the present invention;

FIG. 11 shows schematic waveforms of clock signals θ₀ to θ₃ and outputcurrents IOUT_(A) to IOUT_(D);

FIG. 12 shows waveforms of clock signals θ₀ to θ₃ and partial waveformsof currents IOUT_(A), IOUT_(B) and IOUT_(D);

FIG. 13 shows waveforms for IOUT_(A) to IOUT_(D) as I_(TAIL) increasesand decreases;

FIGS. 14 and 15 present simulation results to demonstrate operation ofan embodiment of the present invention;

FIG. 16 is a schematic diagram of an example implementation of samplingcircuitry embodying the present invention;

FIG. 17 is a schematic diagram of parts of the FIG. 9 ADC circuitry;

FIG. 18 is a schematic diagram useful for understanding operation of thedemultiplexers in FIG. 17;

FIG. 19 is a schematic diagram useful for understanding the principle ofoperation of the FIG. 9 ADC banks;

FIG. 20 is a schematic diagram useful for understanding a possibleapplication of SAR circuitry in embodiments of the present invention;

FIG. 21 is a schematic diagram useful for understanding a possiblelayout of the FIG. 9 ADC circuitry;

FIG. 22 is a schematic diagram useful for understanding the concept ofcalibration techniques employed in embodiments of the present invention;

FIG. 23 is a partial duplication of the waveforms in FIG. 12, withadditional signals representing a VCO timing error;

FIG. 24 is a partial duplication of the waveforms in FIG. 12, withadditional signals representing a threshold voltage error in one of thesampling switches;

FIG. 25 is a schematic diagram of a sampler switch, to show how a trimvoltage may be applied to its body terminal;

FIGS. 26 and 27 present simulation results for a ful implementation ofthe FIG. 9 ADC circuitry; and

FIG. 28 is a schematic diagram of parts of the FIG. 9 ADC circuitry,useful for appreciating a possible refinement.

Before describing detailed embodiments of the present invention, thegeneral concept of embodiments of the present invention will first beexplored.

Much of the previously-considered ADC architectures, as described withreference to FIGS. 1 to 7 above, carry out sampling and demultiplexingbased upon voltages, i.e. in the voltage domain. That is, in sucharchitectures, any information content stored in the signals isrepresented by the voltage level of those signals. In the operation ofsuch circuitry, voltage is the independent variable and current isdependent on the voltage. Put another way, voltage is the cause andcurrent is the effect. Such ADC circuitry is thus consideredvoltage-mode circuitry. In contrast, embodiments of the presentinvention carry out sampling and demultiplexing based upon currents,i.e. in the current domain. That is, in embodiments of the presentinvention, any information content stored in the signals is representedby the current level of those signals. In the operation of suchcircuitry, current is the independent variable and voltage is dependenton the current; current is the cause and voltage is the effect. ADCcircuitry embodying the present invention may thus be considered to becurrent-mode circuitry, although it may of course be incorporated into,or employed in conjunction with, voltage-mode circuitry. Circuitryembodying the present invention may be provided as part or all ofintegrated circuitry, for example as an IC chip. The present inventionmay also extend to circuit boards comprising such IC chips.

In general terms, an embodiment of the present invention takes a currentas the input signal (i.e. a signal whose current magnitude carriesinformation content), and splits that current into n time-interleavedpulse streams, each at 1/n the overall sampling rate, using an n-phasesinewave clock. As information content in embodiments of the presentinvention is represented by the size of the pulses or packets, the sizeof a pulse being measured as an amount of charge since Q=∫Idt, suchcircuitry may be considered “charge-mode” circuitry, and the phrase“current-mode” is to be interpreted accordingly. It is true thatcurrent-mode sampling is known per se, however it will become apparentthat considerable benefits over-and-above such known sampling techniquesresult from embodiments of the present invention disclosed herein.

FIG. 8 is a schematic diagram of sampling circuitry 30. Circuitry 30 hasan input node IN and a plurality of associated output nodes OUT₁ toOUT_(n).

The input node is adapted to receive an input signal as a current 32,the magnitude of the current 32 representing the information carried bythe input signal. Accordingly, although the magnitude of the current 32is steady in FIG. 8 for simplicity, it will be understood that themagnitude could fluctuate, for example to carry a data signal.

The circuitry 30 comprises routing (or steering) means 33 for routingthe received current 32 to different said output nodes (OUT₁ to OUT_(n))over time. In the present case, routing means 33 is operable to routethe current 32 to different output nodes OUT₁ to OUT_(n) in a step-wiseordered fashion and in synchronization with a clock signal (not shown).For example, the routing means 33 is operable to connect the input nodeIN to output node OUT₁ and then to output node OUT₂, and so on and soforth through to output node OUT_(n), before returning to output nodeOUT₁. In this way, the routing means 33 cycles through the output nodessuch that the current 32 is effectively divided up on a time-wise basisinto samples (or parts) 34 ₁ to 34 _(n), received at the output nodesOUT₁ to OUT_(n), respectively.

The routing means 33 may change its connection from one output node tothe next such that no two output nodes are ever connected to the inputnode at the same time. Alternatively, as is true of embodiments of thepresent invention disclosed herein, the routing means 33 may graduallychange its connection from one output node to the next such that thereare periods of time when more than one output node is connected to theinput node.

Circuitry 30 may further comprise generating means (not shown) forgenerating output values based upon the samples or pulses 34 ₁ to 34_(n) received at the output nodes OUT₁ to OUT_(N), the output valuesbeing indicative of input values of parts of the input signal (current32) corresponding to said current samples 34 ₁ to 34 _(n). The circuitry30 may thus be configured to perform analogue-to-digital conversion, forexample if the generating means is operable to output digital outputvalues.

FIG. 9 is a schematic diagram of analogue-to-digital circuitry 40embodying the present invention. Circuitry 40 comprises sampler 42,voltage-controlled oscillator (VCO) 44, demultiplexers 46, ADC banks 48,digital unit 50 and calibration unit 52.

The sampler 42 is configured to perform four-way or four-phasetime-interleaving so as to split the input current I_(IN) into fourtime-interleaved sample streams A to D. For this purpose, VCO 44 is aquadrature VCO operable to output four clock signals 90° out of phasewith one another, for example as four raised cosine signals. VCO 44 mayfor example be a shared 14 GHz quadrature VCO to enable circuitry 40 tohave an overall sample rate of 56 GS/s.

Each of streams A to D comprises a demultiplexer 46 and an ADC bank 48connected together in series as shown in FIG. 9. The sampler operates inthe current mode as in previous embodiments, and, accordingly, streams Ato D are effectively four time-interleaved streams of current pulsesoriginating from (and together making up) input current I_(IN), eachstream having a sample rate one quarter of the overall sample rate.

Continuing the example overall sample rate of 56 GS/s, each of thestreams A to D may have a 14 GS/s sample rate.

Focusing on stream A by way of example, the stream of current pulses isfirst demultiplexed by an n-way demultiplexer 46. Demultiplexer 46 is acurrent-steering demultiplexer and this performs a similar function tosampler 42, splitting stream A into n time-interleaved streams eachhaving a sample rate equal to ¼n of the overall sample rate. Continuingthe example overall sample rate of 56 GS/s, the n output streams fromdemultiplexer 46 may each have a 14/n GS/s sample rate. If n were to be80 or 160 for example, the output streams of demultiplexer 46 may have a175 MS/s or 87.5 MS/s sample rate, respectively. Demultiplexer 46 mayperform the 1:n demultiplexing in a single stage, or in a series ofstages. For example, in the case of n=80, demultiplexer 46 may performthe 1:n demultiplexing by means of a first 1:8 stage followed by asecond 1:10 stage.

The n streams output from demultiplexer 46 pass into ADC bank 48, whichcontains n ADC sub-units each operable to convert its incoming pulsestream into digital signals, for example into 8-bit digital values.Accordingly, n digital streams pass from ADC bank 48 to digital unit 50.In the case of n=80, the conversion rate for the ADC sub-units may be320 times slower than the overall sample rate.

Streams B, C, and D operate analogously to stream A, and accordinglyduplicate description is omitted. In the above case of n=80, circuitry40 may be considered to comprise 320 ADC sub-units split between thefour ADC banks 48.

The four sets of n digital streams are thus input to the digital unit 50which multiplexes those streams to produce a single digital outputsignal representative of the analogue input signal, current I_(IN). Thisnotion of producing a single digital output may be true schematically,however in a practical implementation it may be preferable to output thedigital output signals from the ADC banks in parallel.

Calibration unit 52 is connected to receive a signal or signals from thedigital unit 50 and, based on that signal, to determine control signalsto be applied to one or more of the sampler 42, VCO 44, demultiplexers46 and ADC banks 48. As will become apparent later, it is preferable tocarry out calibration on the sampler 42, which is why the output fromcalibration unit 52 to the sampler 42 is shown as a solid arrow in FIG.9, rather than as a dashed arrow.

Further details regarding the operation, and related benefits, ofcircuitry 40 will become apparent through consideration of FIGS. 10 to27.

FIG. 10 is a schematic circuit diagram of four-phase (i.e. multiphase)current-mode (current-steering) sampler 42. Although in FIG. 9 asingle-ended input signal, current I_(IN), is shown, it will beappreciated that a differential input signal could be employed, forexample to take advantage of common-mode interference rejection.Accordingly, the sampler 42 and demultiplexers 46 and ADC banks 48 couldbe effectively duplicated in circuitry 40 to support such differentialsignaling, however such duplication is omitted from FIG. 9 forsimplicity. Returning to FIG. 10, sampler 42 is configured to receivesuch a differential input current signal, modeled here as a currentsource I_(IN) whose magnitude varies with the input signal.

Because of the differential signaling, sampler 42 effectively has twomatching (or corresponding or complementary) sections 54 and 56 for thetwo differential inputs. Accordingly, there is a first set of outputstreams IOUT_(A) to IOUT_(D) in section 54 and a second set of matchingoutput streams IOUTB_(A) to IOUTB_(D), where IOUTB means IOUT, andwherein IOUT_(A) is paired with IOUTB_(A), IOUT_(B) is paired withIOUTB_(B), and so on and so forth.

Focusing on the first section 54 by way of example (because the secondsection 56 operates analogously to the first section 54), there areprovided four n-channel MOSFETs 58 _(A) to 58 _(D) (i.e. one per streamor path) with their source terminals connected together at a common tailnode 60.

The aforementioned current source I_(IN) is connected between commontail node 60 and an equivalent common tail node 66 of section 56. Afurther current source I_(DC) 62 is connected between the common tailnode 60 and ground supply, and carries a constant DC current I_(DC). Thegate terminals of the four transistors 58 _(A) to 58 _(D) are driven bythe four clock signals θ₀ to θ₃, respectively, provided from the VCO 54.

As mentioned above, section 56 is structurally similar to section 54 andthus comprises transistors 64 _(A) to 64 _(D), common tail node 66 andcurrent source I_(DC) 68.

Operation of the sampler 42 will now be explained with reference toFIGS. 11 to 16.

FIG. 11 shows schematic waveforms for the clock signals θ₀ to θ₃ in theupper graph, and schematic waveforms for the corresponding outputcurrents IOUT_(A) to IOUT_(D) in the lower graph.

The clock signals θ₀ to θ₃ are time-interleaved raised cosine waveformsprovided as four voltage waveforms from the VCO 44. The use of fourclock signals in the present case is due to the four-way-interleavingdesign of ADC circuitry 40, but it will be appreciated that, in anotherembodiment, three or more time-interleaved clock signals could be used,for a three-or-more-way split of the input current signal.

Clock signals θ₀ to θ₃ are 90° out of phase with one another, such thatθ₀ is at 0° phase, θ₁ is at 90° phase, θ₂ is at 180° phase, and θ₃ is at270° phase.

The effect of sampling circuitry 42, under control of clock signals θ₀to θ₃, is that the output currents IOUT_(A) to IOUT_(D) are four trains(or streams) of current pulses, the series of pulses in each trainhaving the same period as one of the clock signals θ₀ to θ₃, and thepulses of all four trains together being time-interleaved with oneanother as an effective overall train of pulses at a quarter of theperiod of one of the clock signals (or at four times the samplingfrequency of one of the clock signals).

FIG. 12 shows more waveforms for the clock signals θ₀ to θ₃ in the uppergraph, and partial waveforms for the currents IOUT_(A), IOUT_(B) andIOUT_(D) (current IOUTc is not shown) in the lower graph, for use inbetter understanding the operation of sampling circuitry 42.

As mentioned above, clock signals θ₀ to θ₃ are time-interleaved raised(substantially) cosine waveforms and are 90° out of phase with oneanother. The clock signals shown are sinusoidal, but need not bestrictly-perfect sinusoids. As will become apparent, in the presentembodiment the shape of the waveforms is more important in the uppermostpart than towards the bottom. For the benefit of further explanation,clock signal θ₀ is highlighted in bold in FIG. 12.

Clock signals θ₀ to θ₃ control the gates of transistors 58 _(A) to 58_(D), respectively, in sampler 42. Accordingly, transistors 58 _(A) to58 _(D) are turned on and then off in sequence, such that as one of themis turning off the next in sequence is turning on, and such that whenone of them is turned fully on the others are substantially turned off.

Because substantially all current entering node 60 via transistors 58_(A) to 58 _(D) must exit that node as current I_(TAIL), then the sum ofcurrents IOUT_(A) to IOUT_(D) at any time must be substantially equal toI_(TAIL)=I_(DC)−I_(IN). The effect of the gate control mentioned aboveis therefore that current I_(TAIL) is steered to pass throughtransistors 58 _(A) to 58 _(D) in the sequence in which thosetransistors are turned on and off, i.e. such that as one of them isturning off and thus starts to carry less of I_(TAIL), the next insequence is turning on and thus starts to carry more of I_(TAIL), andsuch that when one of them is turned fully on, it carries substantiallyall of I_(TAIL) because the other transistors are substantially turnedoff.

This effect is shown in the lower graph of FIG. 12. Only output currentsIOUT_(A), IOUT_(B) and IOUT_(D) are shown for simplicity, however thepattern of waveforms shown continues in a similar way as shown in FIG.11. For comparison with the upper graph of clock signals, the waveformfor output current IOUT_(A) (corresponding to clock signal θ₀) ishighlighted in bold.

In order to gain an understanding of the lower graph in FIG. 12, threepoints, 70, 72 and 74 are indicated on waveform θ₀ and a correspondingthree points 80, 82 and 84 are indicated on waveform IOUT_(A).

At point 70, waveform θ₀ is at its peak value, i.e. at V_(DD), and theother clock signals θ₁ to θ₃ are significantly below their peak value.Accordingly, transistor 58 _(A) is fully on and transistors 58 _(B) to58 _(D) are substantially off. Therefore, at the corresponding point 80,current IOUT_(A) is equal to I_(TAIL) and the other currents IOUT_(B)and IOUT_(D) (and current IOUT_(D) not shown) are substantially equal tozero.

At point 72, which precedes point 70, waveform θ₀ is rising towards itspeak value but has not yet reached its peak value. Also, at point 72,waveform θ₃ is falling from its peak value. Importantly, at point 72clock signals θ₃ and θ₀ have equal values. Therefore transistors 58 _(D)and 58 _(A) are on to the same extent as one another, because theirsource terminals are connected together. At point 72, clock signals θ₁and θ₂ are also equal to one another and are sufficiently low to ensurethat transistors 58 _(D) and 58 _(c) are off. Thus, at this point intime, half of current I_(TAIL) flows through transistor 58 _(D) and halfof it flows through transistor 58 _(A), as indicated by point 82, suchthat IOUT_(D)=IOUT_(A)=(I_(TAIL))/2.

Point 74 is equivalent to point 72, except that at this point it istransistors 58 _(A) and 58 _(B) that are both on. Therefore, atcorresponding point 84, IOUT_(A)=IOUT_(B)=(I_(TAIL))/2.

It will therefore be appreciated that the three points for each currentwaveform (e.g. points 80, 82 and 84 for current waveform IOUT_(A)) arefixed in time relative to the clock waveforms and in magnitude relativeto the current I_(TAIL). That is, taking current IOUT_(A) as an example,at point 80 the current is equal to I_(TAIL) and at points 82 and 84 thecurrent is equal to half I_(TAIL). The location of points 80, 82 and 84is fixed relative to the clock signals θ₀ to θ₃. The same is true forcurrents IOUT_(B) to IOUT_(D). The focus on points 70, 72 and 74demonstrates that for the present embodiment the upper part of the clocksignals is important, and that the lower parts are less important (suchthat, for example, the precise shape of the lower parts is not strictlycritical).

Thus, the current pulses of waveforms IOUT_(A) to IOUT_(D) are all ofthe same shape, and that shape is defined by the raised cosine shape ofthe clock signals. As the value of I_(TAIL) fluctuates with inputcurrent I_(IN), the pulses are simply scaled up or down accordingly.This can be seen from FIG. 13, which shows waveforms for IOUT_(A) toIOUT_(D) as I_(TAIL) increases and then decreases again. The peaks ofthe pulses follow I_(TAIL), and the points where two adjacent pulseshave equal value (i.e. the crossing points) follow half I_(TAIL).

This operation has considerable benefits.

Because the sampling pulses all have the same shape, defined by theraised cosine clock waveforms, the frequency response/roll-off isthereby defined mathematically by the cosine curve and as a result theanalogue bandwidth from the input I_(IN) to the output I_(OUT) is veryhigh, typically greater than 100 GHz. Furthermore, the voltage level atthe tail nodes in the circuitry (e.g. nodes 60 and 66 in FIG. 10) doesnot fluctuate much during operation. By way of explanation, in FIG. 10the switches are nMOS switches, operated in the saturated region, withtheir source terminals tied together to form the tail node concerned.Thus, those switches operate as cascodes with a low input impedance anda high output impedance. Because the voltage level at the tail nodesdoesn't move much with the input signal, those nodes may be consideredto be virtual grounds, and have a reduced sensitivity to parasiticcapacitances at those tail nodes. In FIG. 3, the transistor 22 has todrive capacitor 28. There is thus a high resistance and relatively highcapacitance giving a relatively low bandwidth, and the bandwidth isunpredictable because of the significant mismatch in capacitor 28. Incontrast, in the present embodiment the nMOS switches do not have todrive a capacitor 28; they have a low on resistance and the tail nodeshave a tiny parasitic capacitance (to which the tail node is insensitiveas mentioned above). This gives a high bandwidth that is accurate andrepeatable. In summary, the circuitry of the present embodiment is afast analogue circuit carrying current pulses of a defined shape. Thecircuitry thus has a high bandwidth that is known, repeatable, accurateand constant. This known bandwidth may thus be compensated for with afilter (as mentioned below), for example digitally. As will becomeapparent later, the demultiplexers 46 operate in a similar way to thesampler 42, leading to this high-bandwidth accurate operation over thesampling and demultiplexing circuitry.

Moreover, the sampling circuitry 42 operates by genuinely steering thecurrent I_(TAIL) whose magnitude is dependent upon input signal I_(IN)via the different outputs IOUT_(A) to IOUT_(D). The input currentI_(TAIL) is not copied (in the way that voltages are copied involtage-mode ADCs) from stage to stage (the demultiplexers 46 having asimilar structure and method of operation to the sampling circuitry 42and thus forming subsequent such stages). Instead, it is the actualcurrent I_(TAIL) that is steered or routed through the circuitry. All ofthe current I_(TAIL) passes through to the outputs. It is noted thatalthough the direction of flow of conventional current in this case isfrom output to input, the principles are the same for current flowingfrom input to output, and indeed the graphs of currents IOUT_(A) toIOUT_(D) are shown as positive values (with the direction of thosecurrents shown, e.g. in FIG. 10, as from output to input) to aidconceptual understanding of the operation of the circuitry. In summary,if all of the “output” currents are summed together, the result would bethe same as I_(TAIL).

Another advantage of this operation of the sampler 42 is that theoperation does not suffer from significant input-signal-dependentsampling delays. As will become apparent below, and as briefly discussedabove, the voltage at common tail nodes 60 and 66 can be configured tobe very stable during operation, such that it is only susceptible toaround 50-70 mV ripple. The sampling timing is purely governed by theclock signals θ₀ to θ₃ as explained above (because the source terminalsof the switching transistors are all tied together). Therefore, a changein I_(IN) does not cause any signal-dependent delay, in contrast to thesignal-dependent delay as explained earlier with reference to FIGS. 3 to5. Accordingly, related distortion is also obviated.

Assuming that the clock signals θ₀ to θ₃ are perfect, i.e. free ofamplitude noise and phase noise (jitter), then any errors are mainly(i.e. ignoring insignificant signal-dependent errors) due to mismatchesbetween the switching transistors (and such mismatches are dealt withlater).

Another advantage is that even if there are some sampling errors, duefor example to clock skew and/or mismatches, the input current I_(TAIL)(which is dependent on the input I_(IN)) must still be divided betweenthe output currents IOUT_(A) to IOUT_(D). That is, current is not lost;generally speaking, all current that goes in must come out. Thisprinciple proves particularly useful for calibrating operation of theADC circuitry 40, as will be described in detail below.

Because four time-interleaved sinusoidal clock signals (in this case,raised cosines) are employed in the present embodiment, the 25%duty-cycle pulses required to drive the corresponding four switches(e.g. switches 58 _(A) to 58 _(D) in FIG. 10) are formed even though theclock signals themselves (being sinusoids) naturally have a 50% dutycycle. That is, for an X-way split of the input current signal (X=4,above), it is possible to use 50% duty-cycle sinusoidal clock signals toproduce 100/X % duty-cycle pulses. In contrast, if switched logic-level(hard-switched) clock signals were employed, it would be necessary touse clock signals themselves having a 100/X % (25%, for X=4) duty cycleto produce 100/X % (25%, for X=4) duty-cycle pulses. Therefore, thepresent embodiment is advantageous, particularly when consideringhigh-frequency operation, as 50% duty-cycle clock signals may beemployed (even when X=3 or more).

Yet a further advantage of sampler circuitry 42 is that the gates of thetransistors may be driven directly without requiring an intermediatebuffer such as buffer 24 in FIG. 3. This is because VCO outputs tend tobe sinusoidal and the present circuitry is configured to acceptsinusoidal clock signals. Such direct driving may include intermediateAC coupling, e.g. via a capacitor. With such direct driving, the gatecapacitances of the transistors of the sampler circuitry 42 can beincluded in the VCO 44 design as being part of necessary capacitancewithin the VCO. Thus, the gate capacitances are effectively absorbedwithin the VCO, such that the sampler circuitry 42 operates as if therewere zero gate capacitance. Thus, switching delays due to gatecapacitances are effectively removed. Furthermore, the ability to notemploy buffers such as buffer 24 to generate square (i.e. pulsed orswitched-logic) waves allows associated noise and delay mismatch to beavoided. For example, if quadature square waves were required, then fourbuffers with associated mismatch would be required.

FIG. 14 shows simulation results for sampler circuitry 42. For thepurposes of the simulation, the circuitry 42 has been configured tooperate at 56 GS/s and is supplied with an input current signal I_(IN)having the form of a 1 GHz sinewave.

The uppermost of the four graphs shows the four clock signals θ₀ to θ₃,each being a 14 GHz raised cosine signal, thereby enabling the overalldesired 56 GS/s sampling rate. The second of the four graphs from thetop shows the four output signals IOUT_(A) to IOUT_(D) of section 54 andthe third of the four graphs shows the equivalent traces for outputsignals IOUTB_(A) to IOUTB_(D) of section 56. The lowermost graph showsthe difference between the two components of the differential outputsignal, i.e. generically IOUT−IOUTB. In such differential circuitry, itis the “difference” signal that is the desired output.

FIG. 15 shows an enlarged portion of the simulation results of FIG. 14,in which the pulse shapes and clock signals are more clearly shown.

The resulting fixed pulse shape can be clearly seen. The peaks of thepulses in the middle two graphs follow I_(TAIL) and thus fluctuatearound a DC level defined by the DC current sources 62 and 68. Thecircuitry component values used for the simulation were similar to thoseemployed in the FIG. 16 circuitry (described below), except that theimpedances were halved, the current sources were doubled, and 250resistors were provided in series at the inputs VINP and VINM. Thisenabled the input impedance to remain on target, the voltage swings toremain the same, and the current signals to be doubled. The overallinput 1 GHz signal can be seen as the envelope of the lower graph. Theinput signal is represented both by the peak values of the successivepulses, and by the areas under the successive pulses; because of thedefined shape of the pulses, the peak values double as the areas double,and vice versa.

As already mentioned, ADC circuitry 40 operates in the current domain,i.e. effectively as a current-mode ADC. As a result, the input signalrequired is a current signal. However, typical signals that require useof an ADC or sampler are voltage-domain signals, in which the variableunder examination is a voltage rather than a current. FIG. 16 is aschematic circuit diagram of an example implementation 102 of samplingcircuitry 42, to which it is possible to apply an input differentialvoltage signal.

Implementation 102, similarly to circuitry 42, comprises two sections 54and 56 for the so-called “plus” and “minus” components of thedifferential signals. Section 54 comprises switching transistors 58 _(A)to 58 _(D) connected to common tail node 60, and section 56 comprisesswitching transistors 64 _(A) to 64 _(D) connected to common tail node66, as before.

Implementation 102 basically works by receiving the input voltage signaland by employing resistances to passively convert the received inputvoltage signal into an equivalent input current signal. Implementation102 therefore comprises input terminals 104 (VINP) and 106 (VINM) toreceive the “plus” and “minus” components of the input voltage signal,respectively.

Input terminals 104 and 106 are each connected to a shared terminal 108via a resistor 110 and an inductor 112. Input terminals 104 and 106 arefurther each connected to their corresponding common tail nodes 60 and66 via a resistor 114. Shared terminal 108 is connected via shared DCcurrent source 116 to a reference potential whose voltage level is belowthat of VINP and VINM, and may indeed be negative.

An example set of resistance values are shown in FIG. 16. These valueshave been chosen assuming an example design objective that the inputimpedance looking into each of the common tail nodes 60 and 66 towardsthe switching transistors (sampler switches) is 50Ω, and that the inputimpedance looking into each of the input terminals 104 and 106 is also50Ω as shown in FIG. 16. A further example aim is for the circuitry tohave an analog bandwidth from VIN to IOUT of more than 100 GHz whenimplemented in 65 nm silicon technology.

By setting resistor 110 as 100Ω and resistor 114 as 50Ω, it is possibleto obtain an impedance Z₁ looking into the input terminals 104 and 106towards the respective common tail nodes 60 and 66 of 100Ω at DC and 50Ωat high frequency (e.g. at 100 GHz), an impedance Z₂ looking into theinput terminals 104 and 106 towards the shared terminal 108 of 100Ω atDC and infinity at high frequency, and therefore a total input impedanceZ_(IN) at each input terminal 104, 106 of 50Ω over the full bandwidth.This is because the sampler switches look capacitive at around 100 GHzand are compensated by the series inductors 112 (around 100 pH) in thetail. An advantage of the infinite impedance Z₂ at high frequency isthat shared node 108 effectively serves as an AC ground such that theoperation of the circuitry is shielded from any capacitance in theshared current source 116. Put simply, the inductance 112 isolates anycapacitance in the current source 116 because they compensate for oneanother. Put another way, because the circuit is differential andbalanced, the voltage at node 108 doesn't move much, so there is nosignal on node 108 and that node is insensitive to capacitance.

With this design, it is found that the circuit has the desired 50Ω inputresistance at DC to over 100 GHz, and that this is possible (i.e. thatthe input resistance is stable over frequency) even if the resistanceshave a +/−10% tolerance (not trimmed). That is, the tolerance on theresistances may lead to a slight ripple towards the middle of thefrequency response, but this is considered acceptable in practicalembodiments. It is also found that by applying the voltage input betweenthe two input terminals 104 and 106 as a 600 mVpp signal, by way ofexample, this gives about +/−75 mV ripple at the common tail nodes 60and 66, and about +/−50% current modulation in the sampling switcheswith I_(DC)=6.5 mA.

Advantages of this circuit design thus include that Z_(IN)=50Ω, thecapacitance in the current source 116 has substantially no effect, andthat the bandwidth is very wide (greater than 100 GHz). Furthermore, byemploying the series resistors 110 and 114, the circuitry has inherentESD (electrostatic discharge) immunity, and it is possible to reduce oreven remove ESD diodes at circuitry input pins. Furthermore, the V_(IN)common-mode is around 0 and the current source 116 is below GND (asmentioned above), and only a low current negative supply is needed, e.g.−1V.

To gain a better appreciation of the advantages which arise from the V-Icircuitry of FIG. 16, a comparison may be made with V-I conversioncircuitry which may have appeared more suitable in ADC circuitry,without knowledge of the present invention. In particular, atransconductance stage (i.e. an active V-I conversion circuit) is onetype of V-I conversion circuit which may be considered for use in ADCcircuitry. A transconductance stage can, for example, be useful forproviding a true V-I conversion even if there is expected voltage rippleat the node (e.g. node 60 in FIG. 16) to which the current signal is tobe applied. However, the input impedance of such an activetransconductance stage has a relatively low bandwidth limit (e.g. 10GHz), rendering it unsuitable given the example aims mentioned above(i.e. a desired bandwidth of more than 100 GHz). At high frequency (e.g.above 10 GHz), such a transconductance stage can become a significantnoise and distortion contributor, and can also make input matching poor.In contrast, the present example implementation 102 provides passiveconversion means with substantially constant (i.e. resistive) inputimpedance over frequency (in the example above, there is provided 50Ωinput impedance at DC to over 100 GHz). The example implementation 102enables good input matching and can be used at high frequency withoutbeing a significant noise or distortion contributor. In practicalembodiments of the present invention, it has been found that the rippleat the tail nodes (e.g. nodes 60 and 66 in FIG. 16) may be toleratedbecause a sufficient degree of linearity may still be achieved (e.g. anENOB of 8).

FIG. 17 is a schematic circuit diagram of parts of ADC circuitry 40useful for understanding the structure and operation of thedemultiplexers 46.

For simplicity, only part of the sampler circuitry 42 is shown,employing the implementation 102 of FIG. 16. That is, only the “plus”section 54 is shown, and elements of that “plus” section 54 are omittedto avoid over-complicating FIG. 17.

Regarding the demultiplexers 46, only the demultiplexing circuitry 46for output IOUT_(A) is shown. Similar circuitry may also be provided forthe other seven outputs IOUT_(B) to IOUT_(D), and IOUTB_(A) toIOUTB_(D).

As shown in FIG. 17, demultiplexers 46 in the present embodiment areformed of two stages, namely stages 46A and 46B. The first stage 46Aperforms 1:N demultiplexing, and the second stage 46B performs 1:Mdemultiplexing.

Stages 46A and 46B generally have the same structure as the array ofsampling switches 110 of the sampling circuitry 42. That is, each stagecomprises a plurality of transistors (in this case, n-channel MOSFETs)whose source terminals are connected together at a common tail node.

From the above description of sampling the circuitry 42, and consideringonly the “plus” section 54 by way of example, it will be appreciatedthat the circuitry splits the input currentI_(IN into X time-interleaved trains of pulses, where X=)4 in thepresent embodiment. In the present embodiment, those pulse trains areprovided at outputs IOUT_(A) to IOUT_(D). Sampling circuitry 42 can thusbe thought of as performing a 1:X demultiplexing function. In the sameway, each output from sampler 42 can be further 1:N demultiplexed by astage 46A, and each output of a stage 46A can be further 1:Mdemultiplexed by a stage 46B.

Only one complete demultiplexed path is shown in FIG. 17. That is, inputcurrent I_(IN) is demultiplexed to provide X (X=4 in the present case)outputs IOUT_(A) to IOUT_(D). Each of those outputs is then 1:Ndemultiplexed by a stage 46A, however this is only shown in FIG. 17 inrespect of the left-most output IOUT_(A). Consequently, the outputs fromthat shown stage 46A are outputs IOUT_(A10) to IOUT_(A10(N-1)). Each ofthose outputs (for all stages 46A) is then 1:M demultiplexed by a stage46B, however this is again only shown in FIG. 17 in respect of theleft-most output IOUT_(A10). Consequently, the outputs from that shownstage 46B are outputs IOUT_(A1020) to IOUT_(A102(M-1)). Correspondingoutputs are produced by the other stages 46B.

The sampling circuitry 42 and demultiplexers 46 together carry out a 1:Zdemultiplexing function, where Z=X×N×M. In the present embodiment, X=4,N=8 and M=10. Thus, the present embodiment performs 1:320demultiplexing, which leads to 320 outputs on the “plus” side 54 and acorresponding 320 outputs on the “minus” side 56.

FIG. 18 is a schematic diagram useful for understanding further theoperation of demultiplexers 46. The uppermost trace shows a pulse trainat output IOUT_(A) of the sampling circuitry 42, and the traces belowrepresent corresponding pulse trains of outputs IOUT_(A10) toIOUT_(A1(N-1)) (only IOUT_(A10) to IOUT_(A13) are shown) of a stage 46A.As can be appreciated from FIG. 18, pulse train IOUT_(A) is effectivelysplit up into N pulse trains each at 1/N the sample rate of pulse trainIOUT_(A).

Returning to FIG. 17, it will be appreciated that although the clocksignals θ₀ and θ₃ supplied to sampling circuitry 42 are, in thisembodiment, low-voltage (e.g. having 0 V to 0.8 V clock levels) raisedcosine waveforms (which lead to the related advantages discussedearlier), the specifications for the clock signals applied to stages 46Aand 46B of the multiplexers 46 can be somewhat relaxed in comparison.This is because by that stage the samples have already been taken, andbecause at successive stages the sample rate becomes much reduced. Theclock performance at successive stages therefore becomes less critical.As a result, for example, the clock signals applied to stages 46A and46B could be digital clock signals (i.e. square-wave, pulsed, orswitched-logic signals) rather than sinewaves, and could be highervoltage signals for example with 0 V to 1.2 V clock levels. It is noted,however, that for the purpose of the simulations whose results arepresented herein, sinewave clock signals were applied throughout. Animportant point to note is that the demultiplexers 46 are notsignificant sources of error; they pass the current pulses on with theirrespective areas unchanged, even with reduced bandwidth.

Accordingly, as is apparent from FIGS. 16 and 17, the sampling anddemultiplexing circuitry of the present embodiment forms a series ofnodes connected together in a tree structure. This is true for bothcomplementary parts of the circuitry, looking at, for example, FIG. 16.In FIG. 16, node 60 may be considered a root node of its tree structure,with the next tier of nodes having outputs IOUT_(A) to IOUT_(D), and(moving to FIG. 17) the next tier of nodes having (amongst others notshown) outputs IOUT_(A1020) to IOUT_(A102(M-1)). As mentioned above, thesample-rate reduction from tier to subsequent tier enables the clockspecifications to be relaxed from tier to subsequent tier. Despite thisrelaxtion from tier to tier, because the samples are taken in thesampler circuitry (between the root and first-tier nodes), the samplescan then pass through the subsequent tiers largely unaffected.

Looking back to FIG. 9, the output signals from demultiplexers 46 passinto ADC banks 48. ADC banks 48 are used to produce digital valuescorresponding to the areas of the respective current pulses inputthereto. It is of course not essential for digital values to be output;instead, analog values could be output. Digital output values are simplya useful output format for compatibility with external digital systems,such as processors (e.g. digital signal processors, DSPs) and othercomputing devices.

FIG. 19 is a schematic diagram useful for understanding the principle ofoperation of ADC banks 48. For simplicity, only one output,IOUT_(A1020), of demultiplexers 46 is shown, and consequently the ADCcircuitry 48 shown represents only the ADC circuitry required for thatparticular output. Similar ADC circuitry 48 may be provided for all theoutputs of the demultiplexers 46.

ADC circuitry 48 generally takes the form of a capacitance 150. As shownin FIG. 19, capacitance 150 may be variable in value, such that itsvalue can be trimmed during calibration or during an initial setupphase. Generally speaking, capacitance 150 is employed to convert thecurrent pulses from output IOUT_(A1020) into voltage values V_(OUT).That is, each pulse charges up capacitance 150 to a voltage proportionalto the area of the pulse concerned. This is because the amount of chargein each current pulse is defined by its area (Q=∫I dt), and because thevoltage across the capacitance 150 is defined by that amount of charge Qand the capacitance value C (V=Q/C).

The voltage V_(OUT) for a particular pulse is held across capacitance150 until the circuitry 48 is reset by reset switch 152. Whilst thevoltage V_(OUT) for a particular pulse is held, this analog output valuecan be converted into a digital output value, for example using an ADCcircuit employing a successive-approximation register (SAR). In the caseof differential circuitry, as in the present embodiment, each V_(OUT)will have its complementary V_(OUT), and the pair may be appliedtogether to a differential comparator so that a single digital outputfor that pair is output.

An advantage of this mode of operation is that even if delays areexperienced within the demultiplexers 46, the charge in each pulse willstill make it to the relevant outputs, albeit over a slightly longerperiod. In that case, the voltage V_(OUT) produced from the pulseremains unaffected. To illustrate this point, two examples 154 and 156of the same current pulse are shown in FIG. 19. The first pulse 154represents a case in which minimal delay is experienced. The secondpulse 156 represents a case in which some delay is experienced, forexample due to track capacitance in the circuitry.

Consequently, pulse 156 is stretched in time as compared to pulse 154.Importantly, the area of the two pulses 154 and 156 is substantially thesame, and thus the output voltage V_(OUT) would be the same for both.

Errors (i.e. mismatches) in the values of the capacitances 150 acrossthe ADC circuits in the ADC banks 48 (for example due to tolerancelimitations) may cause errors in the output of ADC circuitry 40. It istherefore advantageous to obtain the lowest capacitance mismatchpossible. Of course, in a practical embodiment, a degree ofcapacitance-value mismatch is inevitable. However, embodiments of thepresent invention have an inherent advantage in this respect overcircuitry in the previously-considered voltage-mode circuitry of FIG. 3,as follows.

Firstly, mismatches in the capacitances 150 have less of an effect thanmismatches in capacitances 28. This is because errors due to mismatchesin capacitances 28 are caused at the initial sampling stage in circuitry20 (where the number of such capacitances 28 is small), however errorsdue to mismatches in capacitances 150 in the present embodiment arecaused at the final ADC stage (where the number of such capacitances 150is large). For each of the X outputs of the sampling circuitry 42 (X=4in the present embodiment), concentrating on one of the “plus” and“minus” halves thereof, there are N×M outputs (N=8 and M=10 in thepresent embodiment) each having a capacitance 150. Therefore, there isan averaging effect as regards mismatches in capacitance 150 inembodiments of the present invention that is not present in circuitry20. It could be expected that errors due to capacitance mismatches inthe present embodiment are, for this reason, reduced by a factor of thesquare root of the number of capacitors per capacitor in FIG. 3, i.e. bya factor of √{square root over (N×M)} (√{square root over (80)} in thepresent case) relative to that in FIG. 3.

Secondly, the capacitors 28 in the FIG. 3 circuitry need to be small toachieve the desired input impedance, and at such a small size themismatch can be large. The example size of capacitor 28 given above was16 fF. In the present embodiment, the capacitances 150 may be relativelylarge, since they are not attached at the inputs. The capacitances 150may, for example, be ten times (10×) larger than the capacitors 28.Taking the above example, capacitances 150 may have a capacitance of 160fF. The reduction in mismatch is found to be approximately proportionedto the square root of the factor increase in area taken up in silicon(assuming silicon implementation), i.e. approximately proportional tothe square root of the factor increase in size of the capacitance value.In the present embodiment, it could be expected that, for this reason,errors due to capacitance mismatches are reduced by a factor of √{squareroot over (10)}.

Taking the above two advantages together, in the present embodiment itcould be expected that errors due to capacitance mismatches for each ofthe sampler channels (in the present example, there are four suchchannels) are reduced by a factor of √{square root over (800)} (√{squareroot over (80)}×√{square root over (10)}), i.e. √{square root over(N×M×I)} (where I is the factor increase in size of capacitance)

FIG. 20 is a schematic diagram useful for understanding a possibleapplication of SAR-ADC (Successive ApproximationRegister-Analogue-to-Digital Conversion) circuitry to circuitry 48 inFIG. 19. Such circuitry could have a cycle of phases of the form: Reset(R); Sample (S); 1; 2; 3; 4; 5; 6; 7 and 8, as shown in FIG. 20. In eachSample phase, a current pulse concerned may be converted into an outputvoltage V_(OUT), and subsequently that voltage V_(OUT) may be turnedinto an 8-bit digital value over the following 8 SAR stages. The nextReset stage then prepares the circuitry for the next current pulse.

FIG. 21 is a schematic diagram useful for understanding a possiblelayout for ADC circuitry 40. Only certain parts of circuitry 40 areshown for simplicity. As can be seen from FIG. 21, and assuming thatX=4, N=8 and M=10, the sampler 42 has four outputs to four demultiplexerfirst stages 46A. Each demultiplexer stage 46A has 8 outputs (this isonly shown for the uppermost demultiplexer first stage 46A) to 8demultiplexer second stages 46B (only one of the 8 demultiplexer secondstages 46B is shown, being for the lowermost output of the uppermostdemultiplexer first stage 46A). Each demultiplexer second stage 46B has10 outputs each to its own ADC. In the way shown in FIG. 21, it ispossible to distribute the switches of the demultiplexer second stages46B so that they are close to their respective sub-ADC circuits of theADC bank 48, thereby to minimize track length between the final switchesand the capacitances 150.

As mentioned above, with reference to FIG. 9, calibration unit 52 isprovided in ADC circuitry 40 to calibrate its operation. In particular,calibration unit 52 is capable of performing such calibration of the ADCcircuitry 40 in use, i.e. without the need to take it “off-line”.

The operation of the calibration unit 52 relies on the principle thatthe sampling circuitry 42 divides up the input current into streams ofcurrent pulses, i.e. that all of the current that is sampled appears inthe pulses at the output. The general idea is that timing errors in theVCO/sampler clocks or switches affect the areas of the current pulses,and therefore the ADC output value. More particularly, as illustrated inFIG. 22, because all the current is divided up into pulses, if one pulseis increased in area (from its expected area in an error-freeenvironment) due to such an error, then another pulse or set of pulsesmust see a corresponding decrease in area because the input current isdivided up into the output currents (without current being added orremoved). Similarly, if one pulse is decreased in area due to an error,then another pulse or set of pulses must see a corresponding increase inarea.

One point to note is that because the clock signals are sinusoidal(which are slow signals, in the sense of rise and fall times, incontrast to fast conventional digital switched-logic or square-wavesignals), the sensitivity of the areas of the current pulses to phase ormagnitude errors in the clock signals, and/or to mismatches between thesampling switches, is maximized. Put another way, taking switch mismatchas an example, the use of sinusoidal clock signals enables the gain fromΔV_(TH) (the change in threshold voltage of a sampling switch,representative of a mismatch) to Δ∫I (the change in area of a resultantcurrent pulse caused by the change in threshold voltage) is maximized.This sensitivity allows these types of error to be detected more easilythan if the clock signals were, for example, switched logic-levelsignals. By way of contrast, if the clock signals were switchedlogic-level clock signals, the sensitivity of the output pulse areas tomismatches would be largely lost.

Statistically speaking, over time one would expect the averaged digitaloutputs of the four ADC banks 48 to be the same as one another. Bycomparing averaged digital outputs of those ADC banks, different typesof error (as briefly mentioned above) may be detected, and as a resultcalibration may be performed to correct or compensate for those errors.This principle will now be explained in relation to FIGS. 23 and 24.

FIG. 23 is a partial duplication of the waveforms shown in FIG. 12. Inthe upper graph, partial waveforms for clocks θ₃, θ₀ and θ₁ are shown.Concentration is placed on clock θ₀ and therefore clocks θ₃ and θ₁ areshown in dashed-line form. The lower graph shows three pulse shapes,labeled P₃, P₀, and P₁ which are intended to schematically represent theaveraged digital output powers corresponding to the outputs of samplercircuitry 42 generated by clocks θ₃, θ₀ and θ₁, respectively. Waveformsθ₃, θ₀ and θ₁ in FIG. 23 represent the effective gate drives for samplercircuitry 42 which would produce equally-sized averaged digital outputpowers P₃, P₀, and P₁.

Also shown in FIG. 23 is an effective gate drive θ_(O-P) which could forexample result from a timing error in the VCO 44. Effective gate driveθ_(O-P) is intended to be a phase-shifted version of the effective gatedrive θ₀ resulting from such a timing error. In this situation, it couldbe expected that the power P₃ would increase, for example by 10%, thatthe power P₀ would stay the same, and that the power P₁ would decreasein line with the increase in P₃, again for example by 10%. Detectingthis pattern of changes in the averaged digital output powers P₃, P₀ andP₁ could therefore indicate the presence of a timing error in VCO 44 inrelation to clock θ₀.

FIG. 24 is a partial duplication of the waveforms shown in FIG. 12, asin FIG. 23. Accordingly, waveforms θ₃, θ₀, θ₁, P₃, P₀ and P₁ in FIG. 24are identical to those in FIG. 23.

Also shown in FIG. 24 is an effective gate drive θ_(0-M), which couldfor example result from a threshold voltage (V_(TH)) error (mismatch) inone of the sampling switches 58 _(A) to 58 _(D) (or 64_(A) to 64 _(D))in sampling circuitry 42. Effective gate drive θ_(0-M) is thereforeintended to be a magnitude-reduced version of effective gate drive θ₀resulting from such a V_(TH) error. In this situation, it could beexpected that the power P₀ would decrease, for example by 20%, and thatthe neighbouring powers P₃ and P₁ would increase equally in line withthat decrease, for example by 10% each. Detecting this pattern ofchanges in the average digital output powers P₃, P₀ and P₁ couldtherefore indicate the presence of such a V_(TH) error.

By way of example, a possible methodology for dealing with the type oferror in FIG. 24 will now be considered. For this example, it will beconsidered that a sampling switch S₀ is found to give rise to an outputpower P₀ and a sampling switch S₁ is found to give rise to an outputpower P₁. In this example, switches S₀ and S₁ are corresponding switchesof a differential pair.

The following equations may be employed to find a measurement of athreshold voltage error ΔV_(TH0) in sampling switch S₀ and a thresholdvoltage error ΔV_(TH1) in sampling switch S₁.

offset error=P ₀ −P ₁ →k(V _(TH0) −V _(TH1))

gain error=P ₀ +P ₁ →k(V _(TH0) +V _(TH1))

-   -   where k is a constant.

${\Delta \; V_{{TH}\; 0}} = \frac{{{gain}\mspace{14mu} {error}} + {{offset}\mspace{14mu} {error}}}{2}$${\Delta \; V_{{TH}\; 1}} = \frac{{{gain}\mspace{14mu} {error}} - {{offset}\mspace{14mu} {error}}}{2}$

The calibration may then be performed taking into account the results ofthis methodology. For example, as will be discussed further below, thebulk and/or gate voltage applied to one or both of sampling switches S₀and S₁ could be controlled to reduce or eliminate the mismatches.

From the above explanation regarding FIGS. 23 and 24, it will beappreciated that different types of error (mismatch) may result indifferent patterns of change in the averaged digital output powers, andtherefore that such different types of error may be detectedindependently of one another or at least compensated for, or calibratedout. Different such types of error may be present simultaneously,however even in this case the various errors may be detected andcompensated for by comparing the powers with one another.

Demultiplexers 46 are similar in structure to sampling circuitry 42 asdiscussed above, and therefore they are similarly susceptible to V_(TH)mismatches. In the same way as a V_(TH) error in a particular samplerswitch (e.g. switch 58 _(A)) can be detected, so an error in aparticular demultiplexer switch (e.g. the switch outputting signalIOUT_(A10) in FIG. 17) may be detected by selecting appropriate digitaloutputs to average and to compare with one another. For example, anaveraged digital output power could be obtained for a particular switchunder investigation by combining all outputs stemming from that switch,and a corresponding output power could similarly be obtained for anotherswitch in the same tier, and these powers (and other similar powers)could be compared as above, to calibrate the switch under investigation.

Following detection of such errors (mismatches), the calibrationcircuitry 52 may be used to adjust operation of the ADC circuitry 40 tocompensate for those errors. Because the errors are detected byaveraging real output signals, the calibration can be carried out“on-line”.

In order to effect the calibration, one possible technique is todirectly control operation of the VCO 44 as indicated in FIG. 9, forexample to adjust the phase offset between the clock signals θ₀ to θ₃until a phase-error effect as in FIG. 23 is eliminated. Anothertechnique is to directly control operation of the switches, primarilyfocusing on the sampling switches in sampling circuitry 42 (since theyare responsible for taking the samples), and, as a secondary measure,focusing on the switches in demultiplexers 46.

One way to directly control operation of the switches (i.e. of thetransistors) is to control the bulk voltage applied to those switches.Another way is to adjust the gate voltages applied to those switches.For example, the clock signals output from the VCO44 may be AC coupled(e.g. via a series capacitor) to the sampler switches, and thus the DCbias may be adjusted. These two methods may be employed in combination.

FIG. 25 is a schematic diagram of such a switch, in which a trim voltageV_(TRIM) is applied to the body terminal. It has been recognized thatmismatch in switching delay may be caused by random variations in thethreshold voltages V_(TH) of transistors. The effect of changing thebulk voltage of a particular switch (transistor) is to change thegate-source voltage at which the transistor changes state. By applyingappropriate trim voltages V_(TRIM) to the body terminals of switchesidentified to be causing errors, their bulk voltages may be varied tocompensate for the effects of random threshold voltage variations ontheir switching points. This technique may also be used to compensatefor other mismatches, i.e. by varying operation of the transistors untilthe effects of those other mismatches are eliminated. A technique ofbulk-voltage variation is disclosed in EP07113379.7 (the entire contentsof which, in particular in relation to FIG. 13 thereof, are incorporatedherein by reference) and this technique may be applied by analogy inembodiments of the present invention.

Calibration can accordingly be carried out “on-line” until the chosenaveraged digital output powers are equal to one another. In this state,the pulses on average have the same area, in which case they have thesame width and are thus equally spaced apart. In this way, sampling skewis removed to a first order approximation.

FIGS. 26 and 27 present simulation results for a full implementation ofADC circuitry 40 configured for the sampling circuitry 42 to perform 1:4demultiplexing and for the demultiplexers 46 to be single-stage 1:4demultiplexers, so that the overall circuitry performs 1:16demultiplexing.

In the simulation, the output current pulses are integrated onto ADCinput capacitors, and the outputs of the demultiplexers 46 arerecombined to create an equivalent single ADC output signal. That singleoutput signal is then subject to FFT processing to produce thefrequency-domain results in FIG. 26. In the upper chart in FIG. 26, theuppermost trace represents a fast silicon process, the middle tracerepresents a typical silicon process, and the lower trace represents aslow silicon process. Those three processes are also represented in theother two charts. The results in FIG. 27 are actual time-domain signaltraces from the simulation.

It can be appreciated that the main cause of bandwidth roll-off is thesample pulse shape as mentioned above. There is small variation over PVT(Process, Voltage, Temperature) tolerance, for example +/−1 dB at 20 GHzas shown in the top chart in FIG. 26. This roll-off can be correctedusing a simple fixed three-tap FIR (Finite Impulse Response) filterproducing close to linear phase including the FIR equalizer, asmentioned previously.

It can be further seen that the output has an ENOB (effective number ofbits) of greater than 8 bits for an input signal frequency of up to 20GHz. This was measured with a full-scale sinewave. Sampler distortionwill fall with signal level.

FIG. 28 is a schematic diagram of parts of ADC circuitry 40 according toone embodiment of the present invention, incorporating a refinement. Forsimplicity, the circuitry in FIG. 28 is presented schematically in asimilar format to that in FIG. 17, except that the resistances areomitted and the switches are represented as crosses. Also forsimplicity, only parts of the sampler circuitry 42 and demultiplexercircuitry 46 are shown, as in FIG. 17. However, both the “plus” and“minus” complementary sections 54 and 56 are shown.

A tail node in the demultiplexer circuitry 46 of section 54 is markedwith numeral 200 and the corresponding tail node in section 56 is markedwith numeral 202. As mentioned earlier, such nodes (like root tail nodes60 and 66 in the samler circuitry 42) generally have a parasiticcapacitance, and therefore a tiny (but nonetheless existent) amount ofcharge can be stored over those parasitic capacitances as packets orpulses of charge pass through those nodes during use.

As the circuitry in FIG. 28 is differential circuitry, it is notspecifically the size of the individual packets of charge that isimportant, but more so the difference between the size of a packet ofcharge passing through section 54 and its corresponding complementarypacket of charge passing through section 56. An amount of charge may beleft behind over the parasitic capacitance of, for example, node 200 asa packet/pulse of charge passes through, and a different amount ofcharge may be similarly left behind at node 202. Thus, these amounts ofresidual charge may affect the sizes of the next pair of pulses thatpass through, and thus be the cause of errors.

To aim to solve this problem, the circuitry of FIG. 28 comprises resetcircuitry 210. Reset circuitry 210 takes advantage of the so-called“down time” that node pair 200 and 202 have between packets of charge.During this down time, the equalization circuitry acts to bring each ofnodes 200 and 202 to the same potential that it was at prior to theprevious packet it carried. In this way, each of those nodes is returnedto the same starting voltage each time before a packet passes through,so that the effect of any residual charge is the same for each packet.This is one way of aiming to keep the difference in potential betweenthe two nodes of the pair the same from cycle to cycle.

In the present embodiment, reset circuitry 210 may, for example, beimplemented as a pair of switches (not shown), connecting theirrespective nodes 200 and 202 to ground via a large capacitance. Thoseswitches may make use of an existing clock signal that is active foranother node at a time when nodes 200 and 202 are intended to beinactive. In that way, nodes 200 and 202 may both be connected to groundtogether to return the residual charge on each of them to a startingvalue during the down time. By returning the residual charge on the twocomplimentary nodes 200 and 202 to a starting value (which may bedifferent for each node), an error that might otherwise have impacted onthe next set of pulses may be substantially prevented.

Reset circuitry 210 may be implemented in many other ways, for exampleit may even be employed to connect nodes 200 and 202 together each time,to equalize the charge on those nodes.

Of course, reset circuitry 210 may be provided for any or all pairs ofcomplementary nodes in the tree structures of Sections 54 and 56, toreset the residual charge on them during their “down time”. Thisincludes all nodes in the tree structure right through the demultiplexercircuitry 46A through to the sub-ADC circuitry 468 & 48 (see FIG. 21),in each case using an appropriate existing (or possibly dedicated) clocksignal to activate the switches concerned during the down time for thepair of nodes concerned. This technique is unpractical between nodes 60and 66, since they do not have “down time” in the way that the othernodes do (which can be appreciated from a comparison of FIG. 13 withFIG. 18).

The present invention may be embodied in many other different forms,within the scope of the appended claims.

The present invention is considered to extend to the present statements:

A.

1. Current-mode circuitry for sampling a current signal, the circuitrycomprising:

-   -   a first node configured to have the current signal applied        thereto;    -   X second nodes conductively connectable to said first node along        respective paths; and    -   steering means for controlling such connections between the        first node and the second nodes so that different packets of        charge making up said current signal are steered along different        said paths over time,    -   wherein:    -   X is an integer greater than or equal to 3; and    -   the steering means comprises:        -   control-signal generating means configured to generate X            time-interleaved sinusoidal control signals; and        -   switching means distributed along said paths and configured            to carry out such control in dependence upon the X            sinusoidal control signals.

2. Circuitry according to statement 1, wherein said control signals areraised generally cosine signals.

3. Circuitry according to statement 1 or 2, wherein said control signalsare substantially evenly time-interleaved relative to one another.

4. Circuitry according to any of the preceding statements, wherein X=4.

5. Circuitry according to any of the preceding statements, wherein saidsteering means is configured such that each of the X paths is controlledby a different one of said X control signals.

6. Circuitry according to statement 5, wherein said steering means isconfigured such that each of the X paths is substantially conductivewhen its control signal is at or around its maximum value, and issubstantially non-conductive when its control signal is at or around itsminimum value.

7. Circuitry according to statement 5 or 6, wherein said steering meansis configured such that when two of said control signals have the samevalue their corresponding paths have substantially the same level ofconductivity.

8. Circuitry according to any of statements 5 to 7, wherein:

-   -   said switching means comprises a transistor per path; and    -   for each path, the transistor concerned is connected such that        its channel forms part of that path and such that it is        controlled by the control signal for that path.

9. Circuitry according to statement 8, wherein:

-   -   said transistors are MOSFETs having gate terminals; and    -   for each path, the gate terminal of the transistor concerned is        connected to receive the control signal for that path.

10. Circuitry according to any of the preceding statements, furthercomprising:

-   -   an input node operable to receive an input voltage signal; and    -   conversion means connected between the input node and the first        node for converting the input voltage signal into the current        signal, the conversion means being configured as passive        circuitry comprising a resistance for performing the conversion.

11. Circuitry according to statement 10, wherein the conversion meanscomprises a resistor network having the resistance and configured suchthat the input node has substantially constant input impedance overfrequency.

12. Circuitry according to statement 11, wherein said first node has acapacitive input impedance, and wherein said network comprises aninductance configured to compensate for the capacitive input impedanceat said first node.

13. Circuitry according to statement 11 or 12, wherein said network isconfigured such that an effect of a voltage fluctuation at said firstnode on conversion of the input voltage signal into the current signalis scaled down.

14. Circuitry according to statement 13, wherein the network has apotential-divider configuration to perform such scaling down.

15. Circuitry according to any of the preceding statements, furthercomprising:

-   -   generating means for generating sample values based upon a        characteristic of respective such charge packets passing through        said second nodes, the sample values being indicative of values        of said current signal corresponding to the charge packets        concerned.

16. Circuitry according to statement 15, further comprising:

-   -   calibrating means operable to combine sample values from one or        more of said second nodes to provide one or more combined sample        values, and to calibrate operation of the circuitry in        dependence upon said combined sample value(s).

17. Circuitry according to statement 16, wherein the calibration meansis operable to combine sample values for a said second node over time tocreate a combined sample value for that node, and to calibrate operationof the circuitry in dependence upon a comparison between that combinedsample value and a reference value.

18. Circuitry according to statement 16 or 17, wherein the calibrationmeans is operable to combine sample values for one said second node overtime to create a combined sample value for that node, and to combinesample values for another said second node over time to create acombined sample value for that other node, and to calibrate operation ofthe circuitry in dependence upon a comparison between those combinedsample values.

19. Circuitry according to any of statements 16 to 18, wherein thecalibration means is operable, for each said second node, to combinesample values for that second node over time to create a combined samplevalue, and to calibrate operation of the circuitry in dependence upon acomparison between those combined sample values.

20. Circuitry according to any of statements 16 to 19, wherein:

-   -   the calibration means is operable to influence operation of the        control-signal generating means and/or the switching means so as        to perform such calibration.

21. Circuitry according to statement 20, wherein the calibration meansis operable to control the phase and/or magnitude of the control signalsso as to perform said calibration.

22. Circuitry according to statement 21, wherein:

-   -   the steering means is configured such that the conductive        connection of each said second node to the first node is        controlled by its said control signal;    -   the calibration means is operable to combine sample values for        one candidate said second node over time to provide a combined        sample value for that node, and to combine sample values for        another candidate said second node over time to provide a        combined sample value for that other node; and    -   the calibration means is operable to control the phase and/or        magnitude of the control signal for one or both of those        candidate nodes to compensate for any difference between the        combined sample values for those candidate nodes.

23. Circuitry according to any of statements 20 to 22, wherein thecalibration means is operable to control a dependency of said switchingmeans on said control signals so as to perform said calibration.

24. Circuitry according to statement 23, wherein:

-   -   the switching means comprises a transistor per path;    -   for each path, the transistor concerned is connected such that        its channel forms part of that path and such that it is        controlled by the control signal for the second node of that        path;    -   the calibration means is operable to combine sample values for        one candidate said second node over time to provide a combined        sample value for that node, and to combine sample values for        another candidate said second node over time to provide a        combined sample value for that other node; and    -   the calibration means is operable to control a gate and/or bulk        voltage for the transistor along the path for one or both of        those candidate nodes to compensate for any difference between        the combined sample values for those candidate nodes.

25. Circuitry according to any of statements 15 to 24, comprising:

-   -   calibrating means operable analyse the or some of the sample        values and to calibrate operation of the circuitry in dependence        upon such analysis.

26. Circuitry according to any of the preceding statements, configuredto have a tree structure, wherein:

-   -   the first node is a root node of the tree structure;    -   the second nodes are first-tier nodes of the tree structure each        of which is conductively connectable directly to the root node;    -   the circuitry further comprises a plurality of subsequent-tier        nodes of the tree structure per first-tier node, each of which        is conductively connectable indirectly to the root node along a        respective path via its said first-tier node; and    -   the steering means is operable to control such connections        between the root node and the subsequent-tier nodes so that        different packets of charge making up said current signal are        steered along different said paths over time.

27. Circuitry according to statement 26, wherein:

-   -   the steering means is operable to employ said sinusoidal control        signals to control connections between the root node and the        first-tier nodes;    -   the control-signal generating means is operable to generate a        plurality of time-interleaved switched-logic control signals;        and    -   the steering means is operable to employ the switched-logic        control signals to control connections between the first-tier        nodes and the subsequent-tier nodes.

28. Circuitry according to statement 27, wherein the control signalsused to control connections between the first-tier of nodes and thesubsequent-tier of nodes have larger peak-to-peak voltages and/or longeron-times than that of the control signals used to control connectionsbetween the root node and the first-tier of nodes.

29. Circuitry according to any of statements 26 to 28, wherein:

-   -   the generating means is operable to generate sample values based        upon a characteristic of respective such charge packets passing        through said subsequent-tier nodes, the sample values being        indicative of values of said current signal corresponding to the        charge packets concerned.

30. Circuitry according to any of the preceding statements, comprising:

-   -   reset means connected to a said node other than the first node,        and operable, during a period between packets for that node, to        tend to bring a voltage potential of that node to a        predetermined value.

31. Circuitry according to any of the preceding statements, wherein thereset means is operable to cause the voltage potential of that node toreturn to the same value between successive packets for that node.

32. Circuitry according to statement 30 or 31, comprising a plurality ofsaid reset means each for a different such node other than the firstnode.

33. Circuitry according to any of statements 1 to 29, configured forsampling first and second complementary said current signals, wherein:

-   -   the circuitry comprises first and second complementary circuit        portions, the first portion for sampling the first current        signal and the second portion for sampling the second current        signal;    -   each said portion comprises such a first node, second nodes and        steering means;    -   the circuitry further comprises reset means connected to at        least one of a said second node of the first portion and the        complementary node of the second portion, and operable, during a        period between packets for that pair of complementary nodes, to        tend to bring a difference in voltage potential between those        two nodes to a predetermined value.

34. Circuitry according to statement 33, wherein said reset means isconnected to both nodes of the pair of complementary nodes.

35. Circuitry according to statement 33 or 34, wherein said reset meansis operable to connect the or both of the nodes of the pair ofcomplementary nodes to a reference potential so as to tend to bring thedifference in potential to a predetermined value.

36. Circuitry according to any of statements 33 to 35, wherein saidreset means is operable to bring the nodes of the pair of complementarynodes to the same potential as one another.

37. Circuitry according to statement 36, wherein the reset means isconnected between said pair of complementary nodes, and is operable toconnect those nodes together so as to bring those nodes to the samepotential as one another.

38. Circuitry according to any of statements 33 to 37, wherein the resetmeans is operable to cause a voltage potential of the or both of thenodes of the pair of complementary nodes to return to the same valuebetween successive packets.

39. Circuitry according to any of statements 33 to 38, comprising aplurality of said reset means each for a different such pair ofcomplementary nodes.

40. Circuitry according to any of statements 33 to 39, wherein each saidportion further comprises:

-   -   generating means for generating sample values based upon a        characteristic of respective such charge packets passing through        the second nodes of that portion, the sample values being        indicative of values of the current signal of that portion        corresponding to the charge packets concerned.

41. Circuitry according to any of statements 1 to 29, configured forsampling first and second complementary said current signals, wherein:

-   -   the circuitry comprises first and second complementary circuit        portions, the first portion for sampling the first current        signal and the second portion for sampling the second current        signal;    -   each said portion comprises such a first node, second nodes and        steering means;    -   each said portion is configured in a tree structure;    -   for each said portion, the first node is a root node of the tree        structure, the second nodes are first-tier nodes of the tree        structure each of which is conductively connectable directly to        the root node of that portion, and that portion further        comprises a plurality of subsequent-tier nodes of its tree        structure per first-tier node, each of which is conductively        connectable indirectly to the root node of that portion along a        respective path via its said first-tier node; and    -   for each said portion, the steering means is operable to control        such connections between the root node and the subsequent-tier        nodes so that different packets of charge making up said current        signal are steered along different said paths over time;    -   the circuitry further comprises reset means connected to at        least one of a said first or subsequent-tier node of the first        portion and the complementary node of the second portion, and        operable, during a period between packets for that pair of        complementary nodes, to tend to bring a difference in potential        between those two nodes to a predetermined value.

42. Circuitry according to statement 41, wherein said reset means isconnected to both nodes of the pair of complementary nodes.

43. Circuitry according to statement 41 or 42, wherein said reset meansis operable to connect the or both of the nodes of the pair ofcomplementary nodes to a reference potential so as to tend to bring thedifference in potential to a predetermined value.

44. Circuitry according to any of statements 41 to 43, wherein saidreset means is operable to bring the nodes of the pair of complementarynodes to the same potential as one another.

45. Circuitry according to statement 44, wherein the reset means isconnected between said pair of complementary nodes, and is operable toconnect those nodes together so as to bring those nodes to the samepotential as one another.

46. Circuitry according to any of statements 41 to 45, wherein the resetmeans is operable to cause a voltage potential of the or both of thenodes of the pair of complementary nodes to return to the same valuebetween successive packets.

47. Circuitry according to any of statements 41 to 46, comprising aplurality of said reset means each for a different such pair ofcomplementary nodes.

48. Circuitry according to any of statements 41 to 47, wherein each saidportion further comprises:

-   -   generating means for generating sample values based upon a        characteristic of respective such charge packets passing through        the subsequent-tier nodes of that portion, the sample values        being indicative of values of the current signal of that portion        corresponding to the charge packets concerned.

B.

49. Current-mode circuitry for sampling a current signal, the circuitrycomprising:

-   -   a first node configured to have the current signal applied        thereto;    -   a plurality of second nodes conductively connectable to said        first node along respective paths;    -   steering means for controlling such connections between the        first node and the second nodes so that different packets of        charge making up said current signal are steered along different        said paths over time;    -   generating means for generating sample values based upon a        characteristic of respective such charge packets passing through        said second nodes, the sample values being indicative of values        of said current signal corresponding to the charge packets        concerned; and    -   calibrating means operable to combine sample values from one or        more of said second nodes to provide one or more combined sample        values, and to calibrate operation of the circuitry in        dependence upon said combined sample value(s).

50. Circuitry according to statement 49, wherein the calibration meansis operable to combine sample values for a said second node over time tocreate a combined sample value for that node, and to calibrate operationof the circuitry in dependence upon a comparison between that combinedsample value and a reference value.

51. Circuitry according to statement 49 or 50, wherein the calibrationmeans is operable to combine sample values for one said second node overtime to create a combined sample value for that node, and to combinesample values for another said second node over time to create acombined sample value for that other node, and to calibrate operation ofthe circuitry in dependence upon a comparison between those combinedsample values.

52. Circuitry according to any of statements 49 to 51, wherein thecalibration means is operable, for each said second node, to combinesample values for that second node over time to create a combined samplevalue, and to calibrate operation of the circuitry in dependence upon acomparison between those combined sample values.

53. Circuitry according to any of statements 49 to 52, wherein:

-   -   the steering means comprises control-signal generating means        configured to generate a set of time-interleaved control        signals, one for each said second node;    -   the steering means comprises switching means distributed along        said paths and configured to carry out such control of the        connections in dependence upon the control signals; and    -   the calibration means is operable to influence operation of the        control-signal generating means and/or the switching means so as        to perform such calibration.

54. Circuitry according to statement 53, wherein the calibration meansis operable to control the phase and/or magnitude of the control signalsso as to perform said calibration.

55. Circuitry according to statement 54, wherein:

-   -   the steering means is configured such that the conductive        connection of each said second node to the first node is        controlled by its said control signal;    -   the calibration means is operable to combine sample values for        one candidate said second node over time to provide a combined        sample value for that node, and to combine sample values for        another candidate said second node over time to provide a        combined sample value for that other node; and    -   the calibration means is operable to control the phase and/or        magnitude of the control signal for one or both of those        candidate nodes to compensate for any difference between the        combined sample values for those candidate nodes.

56. Control circuitry according to any of statements 53 to 55, whereinthe calibration means is operable to control a dependency of saidswitching means on said control signals so as to perform saidcalibration.

57. Circuitry according to statement 56, wherein:

-   -   the switching means comprises a transistor per path;    -   for each path, the transistor concerned is connected such that        its channel forms part of that path and such that it is        controlled by the control signal for the second node of that        path;    -   the calibration means is operable to combine sample values for        one candidate said second node over time to provide a combined        sample value for that node, and to combine sample values for        another candidate said second node over time to provide a        combined sample value for that other node; and    -   the calibration means is operable to control a gate and/or bulk        voltage for the transistor along the path for one or both of        those candidate nodes to compensate for any difference between        the combined sample values for those candidate nodes.

C.

58. Current-mode circuitry for sampling a current signal, the circuitrycomprising:

-   -   a first node configured to have the current signal applied        thereto;    -   a plurality of second nodes conductively connectable to said        first node along respective paths;    -   steering means for controlling such connections between the        first node and the second nodes so that different packets of        charge making up said current signal are steered along different        said paths over time; and    -   generating means for generating sample values based upon a        characteristic of respective such charge packets passing through        said second nodes, the sample values being indicative of values        of said current signal corresponding to the charge packets        concerned,    -   wherein:    -   the steering means comprises control-signal generating means        configured to generate substantially sinusoidal control signals,        and switching means distributed along said paths and configured        to carry out such control in dependence upon the sinusoidal        control signals; and    -   the circuitry further comprises calibrating means operable        analyse the or some of the sample values and to calibrate        operation of the circuitry in dependence upon such analysis.

59. Circuitry according to statement 58, wherein:

-   -   the calibration means is operable to influence operation of the        control-signal generating means and/or the switching means so as        to perform such calibration.

D.

60. Current-mode circuitry for sampling a current signal, the circuitrycomprising:

-   -   a root node configured to have the current signal applied        thereto;    -   a plurality of first-tier nodes each of which is conductively        connectable directly to the root node;    -   a plurality of subsequent-tier nodes per first-tier node, each        of which is conductively connectable indirectly to the root node        along a respective path via its said first-tier node; and    -   steering means for controlling such connections between the root        node and the subsequent-tier nodes so that different packets of        charge making up said current signal are steered along different        said paths over time.

61. Circuitry according to statement 60, wherein:

-   -   the steering means is operable to employ sinusoidal control        signals to control connections between the root node and the        first-tier nodes, and switched-logic control signals to control        connections between the first-tier nodes and the subsequent-tier        nodes.

62. Circuitry according to statement 61, wherein the control signalsused to control connections between the first-tier of nodes and thesubsequent-tier of nodes have larger peak-to-peak voltages and/or longeron-times than that of the control signals used to control connectionsbetween the root node and the first-tier of nodes.

63. Circuitry according to any of statements 60 to 62, furthercomprising:

-   -   generating means operable to generate sample values based upon a        characteristic of respective such charge packets passing through        said subsequent-tier nodes, the sample values being indicative        of values of said current signal corresponding to the charge        packets concerned.

E.

64. Current-mode circuitry for sampling a current signal, the circuitrycomprising:

-   -   a first node configured to have the current signal applied        thereto;    -   a plurality of second nodes conductively connectable to said        first node along respective paths;    -   steering means for controlling such connections between the        first node and the second nodes so that different packets of        charge making up said current signal are steered along different        said paths over time; and    -   reset means connected to a said second node, and operable,        during a period between packets for that node, to tend to bring        a voltage potential of that node to a predetermined value.

65. Circuitry according to statement 64, wherein the reset means isoperable to cause the voltage potential of that second node to return tothe same value between successive packets for that node.

66. Circuitry according to statement 64 or 65, comprising a plurality ofsaid reset means each for a different such second node.

F.

67. Current-mode circuitry for sampling first and second complementarycurrent signals, the circuitry comprising:

-   -   first and second complementary circuitry portions, the first        portion for sampling the first current signal and the second        portion for sampling the second current signal, wherein each        said portion comprises:        -   a first node configured to have the current signal for that            portion applied thereto;        -   a plurality of second nodes conductively connectable to the            first node of that portion along respective paths; and        -   steering means for controlling such connections between the            first node and the second nodes of that portion so that            different packets of charge making up the current signal of            that portion are steered along different said paths of that            portion over time; and    -   reset means connected to at least one of a said second node of        the first portion and the complementary node of the second        portion, and operable, during a period between packets for that        pair of complementary nodes, to tend to bring a difference in        voltage potential between those two nodes to a predetermined        value.

68. Circuitry according to statement 67, wherein said reset means isconnected to both nodes of the pair of complementary nodes.

69. Circuitry according to statement 67 or 68, wherein said reset meansis operable to connect the or both of the nodes of the pair ofcomplementary nodes to a reference potential so as to tend to bring thedifference in potential to a predetermined value.

70. Circuitry according to any of statements 67 to 69, wherein saidreset means is operable to bring the nodes of the pair of complementarynodes to the same potential as one another.

71. Circuitry according to statement 70, wherein the reset means isconnected between said pair of complementary nodes, and is operable toconnect those nodes together so as to bring those nodes to the samepotential as one another.

72. Circuitry according to any of statements 67 to 71, wherein the resetmeans is operable to cause a voltage potential of the or both of thenodes of the pair of complementary nodes to return to the same valuebetween successive packets.

73. Circuitry according to any of statements 67 to 72, comprising aplurality of said reset means each for a different such pair ofcomplementary nodes.

74. Circuitry according to any of statements 67 to 73, wherein each saidportion further comprises:

-   -   generating means for generating sample values based upon a        characteristic of respective such charge packets passing through        the second nodes of that portion, the sample values being        indicative of values of the current signal of that portion        corresponding to the charge packets concerned.

G.

75. Current-mode circuitry for sampling first and second complementarycurrent signals, the circuitry comprising:

-   -   first and second complementary circuitry portions, the first        portion for sampling the first current signal and the second        portion for sampling the second current signal, wherein each        said portion comprises:        -   a root node configured to have the current signal for that            portion applied thereto;        -   a plurality of first-tier nodes each of which is            conductively connectable directly to the root node of that            portion;        -   a plurality of subsequent-tier nodes per first-tier node of            that portion, each of which is conductively connectable            indirectly to the root node of that portion along a            respective path via its said first-tier node; and        -   steering means for controlling such connections between the            root node and the subsequent-tier nodes of that portion so            that different packets of charge making up the current            signal of that portion are steered along different said            paths of that portion over time; and    -   reset means connected to at least one of a said first or        subsequent-tier node of the first portion and the complementary        node of the second portion, and operable, during a period        between packets for that pair of complementary nodes, to tend to        bring a difference in voltage potential between those two nodes        to a predetermined value.

76. Circuitry according to statement 75, wherein said reset means isconnected to both nodes of the pair of complementary nodes.

77. Circuitry according to statement 75 or 76, wherein said reset meansis operable to connect the or both of the nodes of the pair ofcomplementary nodes to a reference potential so as to tend to bring thedifference in potential to a predetermined value.

78. Circuitry according to any of statements 75 to 77, wherein saidreset means is operable to bring the nodes of the pair of complementarynodes to the same potential as one another.

79. Circuitry according to statement 78, wherein the reset means isconnected between said pair of complementary nodes, and is operable toconnect those nodes together so as to bring those nodes to the samepotential as one another.

80. Circuitry according to any of statements 75 to 79, wherein the resetmeans is operable to cause a voltage potential of the or both of thenodes of the pair of complementary nodes to return to the same valuebetween successive packets.

81. Circuitry according to any of statements 75 to 80, comprising aplurality of said reset means each for a different such pair ofcomplementary nodes.

82. Circuitry according to any of statements 75 to 81, wherein each saidportion further comprises:

-   -   generating means for generating sample values based upon a        characteristic of respective such charge packets passing through        the subsequent-tier nodes of that portion, the sample values        being indicative of values of the current signal of that portion        corresponding to the charge packets concerned.

H.

83. Analogue-to-digital conversion circuitry, comprising circuitryaccording to any preceding statement.

84. Integrated circuitry, comprising circuitry according to anypreceding statement.

85. An IC chip, comprising circuitry according to any precedingstatement.

I.

86. A method of sampling a current signal in current-mode circuitry, thecircuitry having a first node configured to have the current signalapplied thereto and X second nodes conductively connectable to saidfirst node along respective paths, the method comprising:

-   -   controlling such connections between the first node and the        second nodes in dependence upon X time-interleaved sinusoidal        control signals so that different packets of charge making up        said current signal are steered along different said paths over        time,    -   wherein:    -   X is an integer greater than or equal to 3.

87. A method of sampling a current signal in current-mode circuitry, thecircuitry having a first node configured to have the current signalapplied thereto and a plurality of second nodes conductively connectableto said first node along respective paths, the method comprising:

-   -   controlling such connections between the first node and the        second nodes so that different packets of charge making up said        current signal are steered along different said paths over time;    -   generating sample values based upon a characteristic of        respective such charge packets passing through said second        nodes, the sample values being indicative of values of said        current signal corresponding to the charge packets concerned;    -   combining sample values from one or more of said second nodes to        provide one or more combined sample values; and    -   calibrating operation of the circuitry in dependence upon said        combined sample value(s).

88. A method of sampling a current signal in current-mode circuitry, thecircuitry having a first node configured to have the current signalapplied thereto and a plurality of second nodes conductively connectableto said first node along respective paths, the method comprising:

-   -   generating substantially sinusoidal control signals;    -   controlling such connections between the first node and the        second nodes in dependence upon the sinusoidal control signals        so that different packets of charge making up said current        signal are steered along different said paths over time;    -   generating sample values based upon a characteristic of        respective such charge packets passing through said second        nodes, the sample values being indicative of values of said        current signal corresponding to the charge packets concerned;    -   analysing the or some of the sample values; and    -   calibrating operation of the circuitry in dependence upon such        analysis.

89. A method of sampling a current signal in current-mode circuitry, thecircuitry having a root node configured to have the current signalapplied thereto, a plurality of first-tier nodes each of which isconductively connectable directly to the root node, and a plurality ofsubsequent-tier nodes per first-tier node, each of which is conductivelyconnectable indirectly to the root node along a respective path via itssaid first-tier node, the method comprising:

-   -   controlling such connections between the root node and the        subsequent-tier nodes so that different packets of charge making        up said current signal are steered along different said paths        over time.

90. A method of sampling a current signal in current-mode circuitry, thecircuitry having a first node configured to have the current signalapplied thereto and a plurality of second nodes conductively connectableto said first node along respective paths, the method comprising:

-   -   controlling such connections between the first node and the        second nodes so that different packets of charge making up said        current signal are steered along different said paths over time;        and    -   for a said second node, and during a period between packets for        that node, tending to bring a voltage potential of that node to        a predetermined value.

91. A method of sampling first and second complementary current signalsin current-mode circuitry, the circuitry having first and secondcomplementary circuitry portions, the first portion for sampling thefirst current signal and the second portion for sampling the secondcurrent signal, wherein each said portion comprises a first nodeconfigured to have the current signal for that portion applied theretoand a plurality of second nodes conductively connectable to the firstnode of that portion along respective paths, the method comprising:

-   -   for each said portion, controlling such connections between the        first node and the second nodes of that portion so that        different packets of charge making up the current signal of that        portion are steered along different said paths of that portion        over time; and    -   for at least one of a said second node of the first portion and        the complementary node of the second portion, and during a        period between packets for that pair of complementary nodes,        tending to bring a difference in voltage potential between those        two nodes to a predetermined value.

92. A method of sampling first and second complementary current signalsin current-mode circuitry, the circuitry having first and secondcomplementary circuitry portions, the first portion for sampling thefirst current signal and the second portion for sampling the secondcurrent signal, wherein each said portion comprises a root nodeconfigured to have the current signal for that portion applied thereto,a plurality of first-tier nodes each of which is conductivelyconnectable directly to the root node of that portion, and a pluralityof subsequent-tier nodes per first-tier node of that portion, each ofwhich is conductively connectable indirectly to the root node of thatportion along a respective path via its said first-tier node, the methodcomprising:

-   -   for each said portion, controlling such connections between the        root node and the subsequent-tier nodes of that portion so that        different packets of charge making up the current signal of that        portion are steered along different said paths of that portion        over time; and    -   for at least one of a said first or subsequent-tier node of the        first portion and the complementary node of the second portion,        and during a period between packets for that pair of        complementary nodes, tending to bring a difference in voltage        potential between those two nodes to a predetermined value.

1-15. (canceled)
 16. Current-mode circuitry for sampling a currentsignal, the circuitry comprising: a first node configured to receive thecurrent signal; X second nodes conductively connectable to the firstnode along respective paths; and a steering circuit configured tocontrol connections between the first node and the second nodes so thatdifferent packets of charge making up the current signal are steeredalong different ones of the paths over time, wherein: X is an integergreater than or equal to 3; and the steering circuit includes: acontrol-signal generating circuit configured to generate Xtime-interleaved sinusoidal control signals; and switching circuitsdistributed along the paths and configured to control the connections onthe basis of the X sinusoidal control signals, wherein the controlsignals are evenly time-interleaved relative to one another.
 17. Thecurrent-mode circuitry of claim 16, wherein X=4.
 18. The current-modecircuitry of claim 16, wherein each of the X paths is controlled by adifferent one of the X control signals and each of the X paths isconductive when the corresponding one of the control signals is at oraround a maximum value, and is non-conductive when the corresponding oneof the control signals is at or around a minimum value.
 19. Thecurrent-mode circuitry of claim 18, wherein when two of the controlsignals have the same value, their corresponding paths havesubstantially the same level of conductivity.
 20. The current-modecircuitry of claim 18, wherein the switching circuits includetransistors corresponding to the paths; and for each of the paths, thecorresponding transistor is connected such that a channel of thecorresponding transistor forms part of the path and such that thecorresponding transistor is controlled by the control signal for thepath.
 21. The current-mode circuitry of claim 20, wherein thetransistors are MOSFETs having gate terminals; and for each of the path,the gate terminal of the corresponding transistor is connected toreceive the control signal for the path.
 22. The current-mode circuitryof claim 16, comprising: an input node configured to receive an inputvoltage signal; and a conversion circuit connected between the inputnode and the first node, the conversion circuit being configured toconvert the input voltage signal into the current signal, the conversioncircuit being a passive circuit including a resistance for convertingthe input voltage signal into the current signal.
 23. The current-modecircuitry of claim 22, wherein the conversion circuit includes aresistor network having the resistance and is configured such that theinput node has constant input impedance over frequency.
 24. Thecurrent-mode circuitry of claim 23, wherein the first node has acapacitive input impedance, and wherein the resistor network includes aninductance for compensating for the capacitive input impedance at thefirst node.
 25. The current-mode circuitry of claim 23, wherein theresistor network is configured such that an effect of a voltagefluctuation at the first node on the conversion of the input voltagesignal into the current signal is scaled down.
 26. The current-modecircuitry of claim 25, wherein the resistor network includes apotential-divider circuit configured such that the effect of the voltagefluctuation is scaled down.
 27. The current-mode circuitry of claim 16,comprising: a generating circuit configured to generate a sample valuefor, and on the basis of characteristic of each of the packets of chargepassing through the second nodes, each sample value being indicative ofa value of the current signal corresponding to its packet of charge; anda calibrating circuit configured to combine the sample values from oneor more of the second nodes to provide one or more combined samplevalues, and configured to calibrate operation of the circuitry on thebasis of the one or more combined sample values, wherein the calibrationcircuit is configured to combine the sample values for one of the secondnodes over time to generate a combined sample value for the one of thesecond nodes, and configured to calibrate the operation of the circuitryon the basis of a comparison between the combined sample value and areference value.
 28. The current-mode circuitry of claim 16, comprising:a generating circuit configured to generate a sample value for, and onthe basis of characteristic of each of the packets of charge passingthrough the second nodes, each sample value being indicative of a valueof the current signal corresponding to its packet of charge; and acalibrating circuit configured to combine the sample values from one ormore of the second nodes to provide one or more combined sample values,and configured to calibrate operation of the circuitry on the basis ofthe one or more combined sample values, wherein the calibration circuitis configured to combine the sample values for a first candidate of thesecond nodes over time to generate a first combined sample value for thefirst candidate of the second nodes, and configured to combine thesample values for a second candidate of the second nodes over time togenerate a second combined sample value for the second candidate of thesecond nodes, wherein the calibration circuit is configured to calibratethe operation of the circuitry on the basis of a comparison between thefirst and second combined sample values.
 29. The current-mode circuitryof claim 16, comprising: a generating circuit configured to generate asample value for, and on the basis of a characteristic of, each of thepackets of charge passing through the second nodes, each sample valuebeing indicative of a value of the current signal corresponding to itspacket of charge; and a calibrating circuit configured to combine thesample values from one or more of the second nodes to provide one ormore combined sample values, and configured to calibrate operation ofthe circuitry on the basis of the one or more combined sample values,wherein the calibration circuit is configured, for each of the secondnodes, to combine the sample values for the each of the second nodesover time to generate a combined sample value for the each of the secondnodes, and configured to calibrate the operation of the circuitry on thebasis of a comparison between the combined sample values for the secondnodes.
 30. The current-mode circuitry of claim 16, comprising: agenerating circuit configured to generate a sample value for, and on thebasis of characteristic of each of the packets of charge passing throughthe second nodes, each sample value being indicative of a value of thecurrent signal corresponding to its packet of charge; and a calibratingcircuit configured to combine the sample values from one or more of thesecond nodes to provide one or more combined sample values, andconfigured to calibrate operation of the circuitry on the basis of theone or more combined sample values, wherein the calibration circuit isconfigured to calibrate the operation of the circuitry by influencingoperation of at least one of the control-signal generating circuit andthe switching circuit.
 31. The current-mode circuitry of claim 30wherein the calibration circuit is configured to calibrate the operationof the circuitry by controlling at least one of a phase and a magnitudeof the control signals.
 32. The current-mode circuitry of claim 31,wherein a conductive connection between each of the second nodes and thefirst node is controlled by the corresponding control signal, and thecalibration circuit is configured to combine the sample values for afirst candidate of the second nodes over time to generate a firstcombined sample value for the first candidate of the second nodes, andconfigured to combine the sample values for a second candidate of thesecond nodes over time to generate a second combined sample value forthe second candidate of the second nodes, wherein the calibrationcircuit is configured to control at least one of a phase and a magnitudeof the control signal for at least one of the first and secondcandidates of the second nodes such that a difference between the firstand second combined sample values for the first and second candidates ofthe second nodes is compensated for.
 33. The current-mode circuitry ofclaim 30, wherein the calibration circuit is configured to control adependency of the switching circuits on the control signals to performthe calibration.
 34. The current-mode circuitry of claim 33, wherein:the switching circuits includes a transistor for each of paths; for eachof the paths, the transistor is connected such that a channel of thetransistor forms part of the corresponding path and such that thetransistor is controlled by the control signal for the second node ofthe corresponding path; the calibration circuit is configured to combinethe sample values for a first candidate of the second nodes over time toprovide a first combined sample value for the first candidate of thesecond nodes, and to combine the sample values for a second candidate ofthe second nodes over time to provide a second combined sample value forthe second candidate of the second nodes; and the calibration circuit isconfigured to control at least one of a gate voltage and bulk voltagefor the transistor along the path for one or both of the first andsecond candidates of the second nodes such that a difference between thefirst and second combined sample values for the first and secondcandidates of the second nodes is compensated for.
 35. The current-modecircuitry of claim 16, wherein the circuitry is configured to have atree structure; the first node is a root node of the tree structure; thesecond nodes are first-tier nodes of the tree structure, each of thefirst-tier nodes being conductively connectable directly to the rootnode; the circuitry includes a plurality of subsequent-tier nodes of thetree structure for each of the first-tier nodes, each of thesubsequent-tier nodes being conductively connectable indirectly to theroot node along a respective path via the corresponding first-tier node;and the steering circuit is configured to control connections betweenthe root node and the subsequent-tier nodes so that different packets ofcharge making up the current signal are steered along different ones ofthe paths over time.
 36. The current-mode circuitry of claim 35,wherein: the steering circuit is configured to employ the sinusoidalcontrol signals to control connections between the root node and thefirst-tier nodes; the control-signal generating circuit is configured togenerate a plurality of time-interleaved switched-logic control signals;and the steering circuit is configured to employ the switched-logiccontrol signals to control the connections between the first-tier nodesand the subsequent-tier nodes.
 37. The current-mode circuitry of claim36, wherein the control signals for controlling the connections betweenthe first-tier nodes and the subsequent-tier nodes have largerpeak-to-peak voltages than the peak-to-peak voltages of the controlsignals for controlling the connections between the root node and thefirst-tier nodes, or longer on-times than the on-times of the controlsignals for controlling the connections between the root node and thefirst-tier nodes.
 38. The current-mode circuitry of claim 35, wherein:the generating circuit is configured to generate a sample value for, andon the basis of a characteristic of, each of the packets of chargepassing through the subsequent-tier nodes, each sample value beingindicative of a value of the current signal corresponding to its packetof charge.
 39. The current-mode circuitry of claim 16 comprising: areset circuit coupled to a node other than the first node, andconfigured, during a period between successive packets of charge for theother than the first node, to bring a voltage potential of the nodeother than the first node to a first value, wherein the reset circuit isconfigured to cause the voltage potential of the node other than thefirst node to return to the same value during the period betweensuccessive packets of charge for the node other than the first node. 40.The current-mode circuitry of claim 16, comprising: a reset circuitcoupled to a node other than the first node, and configured, during aperiod between successive packets of charge for the node other than thefirst node, to bring a voltage potential of the node other than thefirst node to a first value, wherein the circuitry includes a pluralityof reset circuits, each of the plurality of the reset circuits being fordifferent one of the nodes other than the first node.
 41. Thecurrent-mode circuitry of claim 16, wherein the circuitry is configuredto sample first and second current signals of the current signal, thefirst and second current signals being complementary to each other; thecircuitry includes first and second circuits that are complementary, thefirst circuit configured to sample the first current signal, the secondcircuit configured to sample the second current signal; each of thefirst and second circuits includes a said first node, second nodes andsteering circuit; the circuitry includes a reset circuit coupled to atleast one of the second nodes of the first circuit and a complementarynode of the second circuit, the complementary node being complementaryto the at least one of the second nodes; and the reset circuit isconfigured, during a period between successive packets of charge for apair of the at least one of the second nodes and the complementary node,to bring a difference in voltage potential between the pair to a secondvalue, wherein the reset circuit is coupled to both nodes of the pair.42. The current-mode circuitry of claim 16, wherein the circuitry isconfigured to sample first and second current signals of the currentsignal, the first and second current signals being complementary to eachother; the circuitry includes first and second circuits that arecomplementary, the first circuit configured to sample the first currentsignal, the second circuit configured to sample the second currentsignal; each of the first and second circuits includes a said firstnode, second nodes and steering circuit; the circuitry includes a resetcircuit coupled to at least one of the second nodes of the first circuitand a complementary node of the second circuit, the complementary nodebeing complementary to the at least one of the second nodes; and thereset circuit is configured, during a period between successive packetsof charge for a pair of the at least one of the second nodes and thecomplementary node, to bring a difference in voltage potential betweenthe pair to a second value, wherein the reset circuit is configured toconnect at least one of the nodes of the pair to a reference potentialto bring the difference in potential to the second value.
 43. Thecurrent-mode circuitry of claim 16, wherein the circuitry is configuredto sample first and second current signals of the current signal, thefirst and second current signals being complementary to each other; thecircuitry includes first and second circuits that are complementary, thefirst circuit configured to sample the first current signal, the secondcircuit configured to sample the second current signal; each of thefirst and second circuits includes a said first node, second nodes andsteering circuit; the circuitry includes a reset circuit coupled to atleast one of the second nodes of the first circuit and a complementarynode of the second circuit, the complementary node being complementaryto the at least one of the second nodes; and the reset circuit isconfigured, during a period between successive packets of charge for apair of the at least one of the second nodes and the complementary node,to bring a difference in voltage potential between the pair to a secondvalue, wherein the reset circuit is configured to bring the nodes of thepair to the same potential as each other.
 44. The current-mode circuitryof claim 43, wherein the reset circuit is coupled between the nodes ofthe pair and is configured to couple the nodes of the pair together tobring the nodes of the pair to the same potential as each other.
 45. Thecurrent-mode circuitry of claim 16, wherein the circuitry is configuredto sample first and second current signals of the current signal, thefirst and second current signals being complementary to each other; thecircuitry includes first and second circuits that are complementary, thefirst circuit configured to sample the first current signal, the secondcircuit configured to sample the second current signal; each of thefirst and second circuits includes a said first node, second nodes andsteering circuit; the circuitry includes a reset circuit coupled to atleast one of the second nodes of the first circuit and a complementarynode of the second circuit, the complementary node being complementaryto the at least one of the second nodes; and the reset circuit isconfigured, during a period between successive packets of charge for apair of the at least one of the second nodes and the complementary node,to bring a difference in voltage potential between the pair to a secondvalue, wherein the reset circuit is configured to cause a voltagepotential of at least one of the nodes of the pair to return to the samevalue between successive packets of charge for the pair.
 46. Thecurrent-mode circuitry of claim 16, wherein the circuitry is configuredto sample first and second current signals of the current signal, thefirst and second current signals being complementary to each other; thecircuitry includes first and second circuits that are complementary, thefirst circuit configured to sample the first current signal, the secondcircuit configured to sample the second current signal; each of thefirst and second circuits includes a said first node, second nodes andsteering circuit; the circuitry includes a reset circuit coupled to atleast one of the second nodes of the first circuit and a complementarynode of the second circuit, the complementary node being complementaryto the at least one of the second nodes; and the reset circuit isconfigured, during a period between successive packets of charge for apair of the at least one of the second nodes and the complementary node,to bring a difference in voltage potential between the pair to a secondvalue, wherein the circuitry includes a plurality of reset circuits,each of the plurality of the reset circuits being for a different pairof nodes.
 47. The current-mode circuitry of claim 16, wherein thecircuitry is configured to sample first and second current signals ofthe current signal, the first and second current signals beingcomplementary to each other; the circuitry includes first and secondcircuits that are complementary, the first circuit configured to samplethe first current signal, the second circuit configured to sample thesecond current signal; each of the first and second circuits includes asaid first node, second nodes and steering circuit; the circuitryincludes a reset circuit coupled to at least one of the second nodes ofthe first circuit and a complementary node of the second circuit, thecomplementary node being complementary to the at least one of the secondnodes; and the reset circuit is configured, during a period betweensuccessive packets of charge for a pair of the at least one of thesecond nodes and the complementary node, to bring a difference involtage potential between the pair to a second value, wherein each ofthe first and second circuits includes a generating circuit thatgenerates a sample value for, and on the basis of a characteristic of,each of the packets of charge passing through the second nodes of theone of the first and second circuits concerned, each sample value beingindicative of a value of the current signal corresponding to its packetof charge.
 48. The current-mode circuitry of claim 16, wherein thecircuitry is configured to sample first and second current signals ofthe current signal, the first and second current signals beingcomplementary; the circuitry includes first and second circuits that arecomplementary, the first circuit configured to sample the first currentsignal, the second circuit configured to sample the second currentsignal; each of the first and second circuits includes a said firstnode, second nodes and steering circuit; each of the first and secondcircuits has a tree structure; for each of the first and secondcircuits, the first node is a root node of the tree structure, and thesecond nodes are first-tier nodes of the tree structure, each of thefirst-tier nodes being conductively connectable directly to the rootnode; and each of the first and second circuits includes a plurality ofsubsequent-tier nodes of the tree structure for each of the first-tiernodes, each of the plurality of subsequent-tier nodes being conductivelyconnectable indirectly to the root node of the corresponding one of thefirst and second circuits along a respective path via the correspondingfirst-tier node; and for each of the first and second circuits, thesteering circuit is configured to control connections between the rootnode and the subsequent-tier nodes so that different packets of chargemaking up the current signal are steered along different ones of thepaths over time; the circuitry includes a reset circuit coupled to atleast one of the first-tier nodes and the subsequent-tier nodes of thefirst circuit and a complementary node of the second circuit, thecomplementary node being complementary to the at least one of thefirst-tier nodes and the subsequent-tier nodes; and the reset circuit isconfigured, during a period between successive packets of charge for apair of the at least one of the first-tier nodes and the subsequent-tiernodes and the complementary node, to bring a difference in potentialbetween the nodes of the pair to a third value.
 49. The current-modecircuitry of claim 48, wherein the reset circuit is configured to coupleat least one of the nodes of the pair to a reference potential to bringthe difference in potential to the third value.
 50. The current-modecircuitry of claim 48, wherein the reset circuit is configured to bringthe nodes of the pair to the same potential as each other.
 51. Thecurrent-mode circuitry of claim 50, wherein the reset circuit is coupledbetween the nodes of the pair and is configured to couple the nodes ofthe pair together to bring the nodes of the pair to the same potentialas each other.
 52. The current-mode circuitry of claim 48, wherein thereset circuit is configured to cause a voltage potential of at least oneof the nodes of the pair to return to the same value between successivepackets of charge for the at least one of the nodes of the pair.
 53. Thecurrent-mode circuitry of claim 48, comprising a plurality of the resetcircuits, each of the plurality of the reset circuits being for adifferent pair of nodes.
 54. The current-mode circuitry of claim 48,wherein each of the first and second circuits includes a generatingcircuit that generates a sample value for, and on the basis of acharacteristic of, each of the packets of charge passing through thesubsequent-tier nodes of the one of the first and second circuitsconcerned, each sample value being indicative of a value of the currentsignal corresponding to its packet of charge.
 55. Analogue-to-digitalconversion circuitry, comprising the current-mode circuitry according toclaim
 16. 56. Integrated circuitry, comprising the current-modecircuitry according to claim
 16. 57. An IC chip, comprising thecurrent-mode circuitry according to claim
 16. 58. A method of sampling acurrent signal in current-mode circuitry, the circuitry having a firstnode configured to receive the current signal and X second nodesconductively connectable to the first node along respective paths, themethod comprising: controlling connections between the first node andthe second nodes on the basis of X time-interleaved sinusoidal controlsignals so that different packets of charge making up the current signalare steered along different ones of the paths over time, wherein X is aninteger greater than or equal to 3.